The weapons, including an 18-inch fragment of a Grad-type Katyusha rocket seen by The Associated Press, are believed to be reaching blockaded Gaza through a clandestine network: by sea from Sudan to Egypt's Red Sea ports and then by land through the Sinai desert to tunnels that cross into the coastal strip, according to the officials.
Trucks and airplanes also carry Iranian-made rockets across the Syrian-Lebanese border, according to the officials, who spoke on condition of anonymity under military restrictions.
Hezbollah guerrillas bombarded Israel with nearly 4,000 rockets in their 2006 war. After recent clashes across Lebanon killed at least 67 people, Hezbollah forced the weakened Lebanese government into concessions that could free the guerrilla group to bring in even more rockets.
The Israeli claims - although expressed privately by security authorities - have not been backed up by a public display of evidence, leading some analysts to question the extent of Iranian involvement on Israel's borders. Iran, Hezbollah and Palestinian militants all deny an Iranian arms connection, though some Hezbollah militants privately acknowledge getting arms from Tehran.
But it's clear Iran has sharply increased its regional profile after the fall of arch-rival Saddam Hussein in Iraq and the rise of a Shiite-led government in Baghdad with close ties to Tehran.
The Israel connection isn't new. Six years ago, Israeli naval commandos captured a ship in the Red Sea, the Karine A, that Israel said was carrying 50 tons of missiles, mortars, rifles and ammunition from Iran to the Palestinians.
Egypt has publicly denounced suspected Iranian involvement in the conflict. Its foreign minister, Ahmed Aboul Gheit, accused Iran of being behind Hamas' violent takeover of Gaza nearly a year ago.
Experts think Iran's wider aim is to indirectly pressure Israel. Establishing proxies on Israel's borders raises the price of any possible Israeli attack on Iranian nuclear facilities and makes it tougher for Israelis and Palestinians to forge a peace pact, they say.
Israel has not explained why it hasn't publicly released serial numbers and other rocket markings to prove Iranian interference. Some analysts suggest Israel is making unsubstantiated claims to keep up world pressure on Iran to trim its nuclear ambitions.
Hamas and other militant factions in Gaza have been firing crude projectiles into southern Israel for years. Israel's high-tech military hasn't been able to stop the rockets, which are increasingly striking closer to Israel's heartland.
Israeli military ballistics experts have identified Iranian origins from paint, toolwork and Latin lettering on weapon fragments, senior Israeli security officials said. Similar rockets produced in Eastern Europe look different, they said.
The Grad fragments seen by the AP had threading indicating it was made in Iran, a security officer said. Hezbollah fired similar rockets at Israel during the 2006 war, he added.
Longer-range missiles have hit the town of Ashkelon in recent months, strengthening the suspicion that the militants are being supplied by Iran.
"Iran is, unfortunately, very much involved in supporting the buildup of the Hamas military machine in Gaza, whether it's in training Hamas operatives in different areas of technical know-how, whether it's in just funding them, whether it's supplying them with munitions, whether it's giving them capabilities to upgrade indigenous defense capabilities," government spokesman Mark Regev said.
The arsenal of Iranian-made weapons improved after Hamas militants blew open the border fence between Gaza and Egypt in January, allowing more arms to enter. Some of the new rockets can travel 25 miles - just 12 miles short of Tel Aviv, the Israelis say.
One rocket that recently slammed into Israel carried a 170mm warhead, officials said. Previous rockets carried 120mm warheads.
An April report by the Intelligence and Terrorism Information Center, an Israeli think tank with close ties to the defense establishment, pointed to a number of recent findings it claimed were examples of Iranian technology used by Gaza militants.
Rockets fired into southern Israel during a recent flare-up in fighting had engines divided into four 20-inch parts "to make it easier for the terrorist organizations to smuggle the rockets into the Gaza Strip by dismantling the sections," the report said.
"You then no longer require or are relying on a technician in Gaza to pull the rocket together from indigenous supplies, but actually are getting the entire rocket in, which would mean a more professionally prepared and put-together weapon," Riedel said.
Abu Obeida, a Hamas military leader in Gaza, says Israel is claiming the Iranian connection to mobilize international support for an attack on Gaza.
"The Gaza Strip was always capable of manufacturing the tools it needs for its resistance," added Hamas' deputy leader, Moussa Abu Marzouk, in a telephone interview from his base in Damascus, Syria.
Iranian officials did not respond to calls seeking comment. In the past, Iran has acknowledged giving millions of dollars to Hamas but denied supplying arms.
Experts are divided on whether Iran is directly providing weapons to Hamas.
John Pike, director of GlobalSecurity.org, an Alexandria. Virginia-based defense research firm, said he "would not need a great deal of convincing," given Iran's history of support for Palestinian militants.
But David Hartwell, Middle East and North Africa editor at Jane's Country Risk, a London publication, said that while Israel's claims "are not outside the realm of possibility ... without any evidence to back it up, it's very difficult to substantiate any of the Israeli allegations."
Meir Javedanfar, an Iran expert based in Israel, thinks Iran is more likely financing the purchase of the more sophisticated weapons Israel has seen.
"We haven't seen any kind of evidence by Israel, like serial numbers of weapons, or any traces, to prove they are Iranian-made," he said. "Money - I think that's the highest probability."
Tehran is thought to have pledged at least US$300 million (euro190 million) to Hamas, but it is not clear how much money has been delivered. Israeli security officials cite different numbers, ranging to tens of millions of dollars.
Israel, like many of its Western allies, does not believe Iran's assertions that its nuclear program is meant only to produce energy. Iranian President Mahmoud Ahmadinejad has heightened Israel's alarm with repeated calls for the destruction of the Jewish state.
Iran needs a deterrent to an Israeli attack on its nuclear facilities, said Menashe Amir, director of Israel Radio's Farsi service. "So it built Hezbollah on (Israel's) northern border, and is now cultivating Hamas and the Islamic Jihad in Gaza."
One senior Israeli security official said Iran tightened its ties with Hamas and Islamic Jihad after the Lebanon war because the Iranians "understood that Hezbollah cannot be activated as frequently as they want."
One possible brake on any future Hezbollah action against Israel would be a successful conclusion of newly resumed Syrian-Israeli peace talks. Hezbollah also enjoys support from Damascus, and a peace deal between Israel and Syria could reduce Syrian backing for Hamas as well.
In the meantime, indirect cease-fire talks between Israel and Hamas have been dragging on for months - increasing the prospect that Israel would launch a threatened major military operation there.
A report in March by U.N. Secretary-General Ban Ki-moon cited Israeli claims that Hezbollah now commands 10,000 long-range rockets and 20,000 shorter-range ones. Other Israeli officials have said the number is much higher. Before the 2006 conflict, Israel estimates, Hezbollah had 14,000 rockets.
But it's not just a matter of numbers, says Israel's military intelligence chief, Amos Yadlin. He told the Haaretz newspaper in an interview published last week that Hezbollah now has weapons that "cover large areas of Israel."
Senior Israeli defense officials say Hezbollah's new Iranian rockets can fly 185 miles. In 2006, the farthest any flew was 45 miles inside Israel.
If another war were to break out, Israel "will face a stronger Hezbollah," Yadlin said.