Updated 1:42 p.m. ET
TRIPOLI, Libya - Troops loyal to Muammar Qaddafi unleashed heavy shelling Friday on Misrata, pushing troops and tanks into the rebel-held western city, a witness said, while NATO officials struggled to overcome differences over its mission to dislodge the defiant Libyan leader.
Elsewhere in Libya, NATO warplanes struck Qaddafi's hometown of Sirte in the east, Libyan TV said. In the capital of Tripoli, there were reports of heightened security measures in an apparent attempt to prevent anti-government protests.
President Barack Obama said in an interview with The Associated Press that a military stalemate exists in Libya, but the U.S. and NATO have averted a "wholesale slaughter" and that Qaddafi is coming under increasing pressure to leave.
A helicopter circled over Misrata for several hours, apparently spotting targets for artillery in Libya's third-largest city, in defiance of the NATO-enforced no-fly zone. Pro-Qaddafi forces bombarded the city with fire from tanks, artillery and rockets, residents said.
Eight bodies of civilians were taken to a hospital, said the resident, who spoke on condition he be identified only by his given name, Abdel-Salam, for fear of retaliation. Abdel-Salam said he believes there are additional casualties among the fighters.
Qaddafi's forces are using heavy artillery -- including internationally banned cluster bombs -- and targeting residential neighborhoods with the weapons, the New York Times reports, citing witnesses on the ground.
Secretary of State Hillary Clinton said she was not specifically aware of the use of cluster bombs, but told a press conference, "I'm not surprised by anything that Colonel Qaddafi and his forces do" and called the information "worrying."
Rebels in Misrata have complained that NATO is not doing enough to help them keep Qaddafi's forces at bay.
"Where is NATO? ... This is what people are saying," Abdel-Salam said. "Their top mission is to protect civilians, and Misrata is the No. 1 city in Libya that needs protection for the civilians."
Another Misrata resident said Qaddafi loyalists have been firing randomly in the city, forcing people to leave their homes. Once a building is empty, it is being taken over by government troops, said the resident, who spoke on condition of anonymity for fear of retribution.
He said rebel-held neighborhoods are becoming increasingly crowded. "Now you can find houses with more than 10 families in one house," he said.
Qaddafi's troops have been targeting groups of civilians, including people standing in line outside a bakery, he said.
Qaddafi's troops have continued to attack rebel positions as part of a deadlocked civil war sparked two months ago by anti-government protests. The international community stepped into the conflict a month ago, with NATO unleashing airstrikes on Qaddafi-linked military targets.
Libyan TV said airstrikes targeted Sirte, although it did not provide details. Explosions were also heard from what appeared to be NATO strikes against Qaddafi's forces near the coastal town of Brega.
Qaddafi controls the west of the country, while the rebels hold much of the east, with the front shifting back and forth.
There has been mounting international pressure on Qaddafi to step aside after 42 years in power. The leaders of the U.S., Britain and France pledged Friday to maintain the campaign until Qaddafi goes "for good," a show of unity amid European complaints that Washington is not carrying its full weight in the crisis.
Obama, British Prime Minister David Cameron and French President Nicolas Sarkozy wrote in a joint newspaper opinion article that while their mandate under a U.N. Security Council resolution does not include removing Qaddafi by force, "it is impossible to imagine a future for Libya with Qaddafi in power."
"So long as Qaddafi is in power, NATO must maintain its operations so that civilians remain protected and the pressure on the regime builds," the three leaders wrote.
Obama said in the AP interview that he doesn't see a need to resume direct U.S. participation in enforcing the no-fly zone, saying it's assisting with intelligence, jamming and refueling. He acknowledges there was essentially a stalemate there but added that the NATO operation is less than a month old.
Qaddafi is "getting squeezed in all different kinds of ways" and is running out of money and supplies, Obama said. He added that he is confident Qaddafi ultimately will be forced to surrender power and that there is no need for a change in U.S. policy at this time.
Anti-government protests have been rare in Qaddafi-controlled areas, after the government clamped down hard shortly after the outbreak of mass protests elsewhere in the country in February.
In the capital of Tripoli, participants in a Facebook group said snipers were deployed Friday on rooftops in the Tajoura neighborhood and that security was tight around mosques.
Sustained bursts of gunfire could be heard in Tripoli on Friday afternoon, but the circumstances of the shooting were unclear. The government has restricted the movement of foreign journalists in Tripoli, barring them from leaving their hotel without an escort.
About two dozen Qaddafi opponents staged a march Thursday and distributed a video of it, showing activists their faces hidden by scarves gathering in a rural area and holding up rebel flags. In a statement, one of the protesters spoke of an "intense grip of security" by the government, but said that "victory is near."
On Thursday, Al-Sadek al-Ghariani, a top Muslim cleric in Libya, said in a video posted on Facebook that it was a religious duty to join protests on Friday. In February, he issued two fatwas calling for anti-Qaddafi protests and then went into hiding. Qaddafi forces apparently are trying to find him.
The latest air strikes followed new shows of defiance by Qaddafi on Thursday and by his daughter, Aisha, who rallied a crowd early Friday from a balcony at her father's compound that was hit by U.S. warplanes 25 years ago.
Aisha pumped her right fist as she led the crowd in chants from the second-floor balcony of the badly damaged Bab Aziziyah compound, targeted by U.S. warplanes in 1986. "Leave our skies with your bombs," she said, referring to NATO airstrikes on Tripoli just hours earlier.
Friday was the 25th anniversary of the U.S. attack on targets in Libya, including Qaddafi's Bab Aziziyah compound in Tripoli. Those airstrikes came in retaliation for the April 5, 1986, bombing of a Berlin disco that killed two U.S. servicemen. Dozens of people were killed in those airstrikes, including Qaddafi's adopted daughter.
The anniversary highlighted Qaddafi's long and troubled history with the West. In the 1980s, he was seen as a sponsor of terrorism, including the 1988 bombing of Pan Am Flight 103 over the Scottish town of Lockerbie, arming militant groups such as the Irish Republican Army, and the Berlin disco bombing, which was blamed on Libya's secret service.
Hundreds rallied at Bab Aziziyah late Thursday and early Friday, chanting pro-Qaddafi slogans, such as "Only Allah, Moammar and Libya," and "The people want Moammar as their leader."
The crowd erupted in cheers when Aisha appeared on the balcony.
"Let me go back to the past when I was a child, when I was 9 years old, in this house," she said. "A rain of missiles and bombs. They tried to kill me. They killed dozens of children in Libya."
"Now, after 25 years, the same missiles, the same bombs, rain on our children's heads," she said.
"We are a people that cannot be defeated," she said.