(CBS/AP) For the third time in as many months, the United States on Monday warned it has credible intelligence of a terrorist threat to commercial aircraft.

It was the second recent warning to highlight worries about Saudi Arabia in particular, a move that may irk Saudis spending millions to improve their country's image in the United States.

Saudi Arabia has spent $17.6 million on public relations, advertising and lobbying since the Sept. 11, 2001, attacks, hoping to convince Americans it is committed to fighting terrorism although 15 of the 19 hijackers were Saudi citizens, Justice Department records show.

Monday's State Department advisory suggests Americans "defer non-essential travel to Saudi Arabia."

"The U.S. government continues to receive indications of terrorist threats aimed at American and Western interests, including the targeting of transportation and civil aviation," the department said in a statement.

"There is credible information that terrorists have targeted Western aviation interests in Saudi Arabia," the warning continued. "American citizens in Saudi Arabia should remain vigilant, particularly in public places."

Britain's Foreign Office said Friday it believed that "terrorists may be in the final phases of planning attacks" in Saudi Arabia.

The warnings continued concerns about threats to commercial aircraft that have existed since Sept. 11, and were only reinforced by the attempt to shoot down an Israeli jetliner in Kenya with missiles last November.

In September, a Homeland Security advisory cited "a growing body of credible intelligence" that suggests al Qaeda "continues to develop plans for multiple attacks against targets in the U.S. involving commercial aircraft, with some plans calling for hijacking airliners transiting near or flying over the continental United States — but not destined to land at U.S. airports."

In August, the United States was joined by Australia and Britain in warning of a possible threat to Saudi flights. British Airways suspended flights to the Arab kingdom.

Also in September, federal officials barred five Saudi pilots from flying into the United States for "security concerns." Officials said only that "something" in the backgrounds of five of the pilots raised enough of a red flag to create a concern that "they shouldn't be flying into the U.S."

Saudi Arabia has been buffeted by accusations of going soft on terror, suspicions fueled by a Congressional report in July that included 28 pages of blacked-out material on Saudi links to the 2001 terrorist attacks on New York and Washington.

But television ads on CNN, ESPN, MSNBC and Fox News since May 2002 have depicted the Saudis as a modern nation aligned with American interests. In a two-week period last October, 1,541 Saudi-sponsored ads ran on American television.

The Saudi strategy to win the hearts and minds of American citizens and lawmakers is clear: spend large amounts on media advertising, book time with television's news shows, lobby congressional leaders and monitor policies coming out of Washington.

The Saudis have hired three well-connected Washington lobbying and law firms to advance their case in the capital. One firm, paid $420,000 so far this year, is headed by former Rep. Thomas Loeffler, a top contributor to President Bush when Mr. Bush was governor of Texas and a major fund-raiser in Mr. Bush's presidential campaigns.

Foreign Agent Registration Act filings reviewed by The Associated Press show that Loeffler and his wife contributed $8,000 this year to the re-election campaign of Sen. Richard Shelby, R-Ala., a leading critic of Saudi Arabia's commitment to fight terrorism. The couple also sent $2,000 to the Mr. Bush campaign.

Most of the Saudi money — about $16 million — went for television, radio and print ads in the top 20 markets across the country. The ads, according to Saudi officials, were designed to impress upon the American people that Saudis really are allies against terror.

"The main purpose of the campaign is to get our voice out to the American people" and counter voices critical of the Saudi government, religion and culture, said Nail al-Jubeir, spokesman for the Saudi Embassy.

An open letter to Mr. Bush from Saudi Crown Prince Abdullah, Saudi Arabia's de facto ruler due to the illness of his brother, King Fahd, expressed sympathy and solidarity to the United States in the newspaper USA Today on the first anniversary of the attacks. Subsequent ads ran in Time, Newsweek and U.S. News & World Report news magazines.

Along with the ad campaign, al-Jubeir joined Saudi Ambassador Prince Bandar bin Sultan and Foreign Minister Prince Saud al-Faisal on a tour of the United States that hailed the Saudi commitment and contribution to the war against terror.

Al-Jubeir said the Saudi effort to convince skeptical members of Congress of his country's anti-terror commitment is not lobbying.

"We're simply trying to communicate: this is what we've done; this is what we're doing; and this where we're going in the future," he said.

The Saudi campaign, however, hasn't quieted its critics.

"We can't get over the fact that 15 of the 19 hijackers were from Saudi Arabia," said Shelby, a former chairman of the Senate Intelligence Committee. He said the Saudis have a long way to go to prove their commitment to fighting terrorism.