Numerous senior Iraqi officials have fled into Syria over the last two weeks, with some moving on to two third countries and "a handful" still in hiding in Syria, a senior Bush administration official said yesterday. They include Iraq's top nuclear scientist, who turned himself in to U.S. authorities over the past several days after making his way to another Persian Gulf country.

Jaffar Dhai Jaffar, who founded and led Iraq's clandestine nuclear weapons program and was one of deposed Iraqi president Saddam Hussein's three top science advisers, could provide valuable information about the status of Iraq's proscribed weapons programs. Another senior official said Jaffar is in U.S. military custody at an undisclosed location in the Gulf region.

Jaffar and Lt. Gen. Amir Saadi, an Iraqi scientist who surrendered to U.S. troops Saturday in Baghdad, "know, between the two of them, everything about the country's nuclear, biological, chemical and missile programs," said David Albright, a former U.N. weapons inspector who is now president of the Institute for Science and International Security. Jaffar, he added, "is the best scientist Iraq ever produced."

U.S. intelligence officials confirmed Albright's view of Jaffar but said he had not yet proved helpful under U.S. questioning. If he cooperates, one official said, "he could tell us the whereabouts" of prohibited weapons as well as the countries or groups that supplied Iraq with weapons components and knowledge.

U.S. troops have not found chemical or biological weapons, or nuclear weapons components, in Iraq, although the presence of such weapons was cited by President Bush as one of the main justifications for the war.

As the hunt for members of the ousted Iraqi government continued, the fate of most of them, including Hussein, remained a mystery. One senior official said yesterday that U.S. intelligence reports seem to indicate the former Iraqi president "was more dead than alive," though the official added there was no absolute evidence that would prove it. "There are all sorts of reports saying he's dead," the official said.

U.S. military teams have begun digging through a complex in southern Baghdad that was the target of an airstrike by U.S. Tomahawk cruise missiles and 2,000-pound bunker-penetrating bombs on March 20, the first night of the war. CIA officials believed Hussein and his sons were spending the night at the complex and did not exit before the bombs hit, though there were conflicting reports in the weeks that followed over whether the Iraqi leader had been killed or wounded or had escaped unharmed.

On April 7, U.S. warplanes struck a building in the upscale Mansour neighborhood of Baghdad, where intelligence officials received what they said was a reliable tip that a man who matched Hussein's description was holding a meeting with senior Iraqi intelligence officials. It remains unclear whether any Iraqi leaders were at the site.

Defense Secretary Donald H. Rumsfeld and other Pentagon officials have charged for several days that Syria was allowing Iraqi leaders to cross the border to escape. U.S. officials yesterday were not precise about how many Iraqi officials have fled to Syria, though a defense official said Hussein's wife is believed to be among them.

The official said that a large convoy of several dozen vehicles crossed into Syria from Iraq during the first week of the war and that it is believed to have been carrying Hussein's wife and other government officials.

"There's certainly a lot of circumstantial evidence that absolutely high-level people are there," an administration official said. However, there is no conclusive list of Iraqi officials in Syria because the intelligence is inconclusive. "We have scraps of information," another senior official said.

Two weeks ago, U.S. troops began blocking the main roads out of Iraq to Syria, but the 375-mile-long border includes hundreds of small roads used for decades by smugglers to move contraband goods. "We still don't control all the back roads," said one administration official. "They could still escape that way."

The two Iraqi scientists now in U.S. custody amount to "a very good catch," said Khidhir Hamza, the head of Iraq's nuclear weapons program until he fled the country in 1994. Jaffar, in particular, "is a very well-connected member of the cabinet," he said.

In his book, "Saddam's Bombmaker," Hamza describes Jaffar as "a willowy genius" Hussein arrested and jailed in 1979 for questioning Iraq's nuclear weapons program.

"To give him a taste of what could come," Hamza wrote, "Jaffar was strapped to a dungeon wall and forced to watch as other men were tortured," including a colleague, Hussein Shahristani. Jaffar "recanted and returned to work."

Officially, Jaffar headed Iraq's nuclear program only until 1991, when he dropped out of sight. It is believed he then "launched Iraqi's underground nuclear program," Albright said.

Saadi, who worked in the Iraqi chemical weapons program in the 1980s and 1990s under Hussein's son-in-law, last year became the main liaison with U.N. weapons inspectors. Upon his surrender, Saadi told a German television network that Iraq no longer possessed weapons of mass destruction, a position he maintained before the war in his contacts with the U.N. inspection teams.

The Bush administration charged this year that Iraq had secretly reconstituted its nuclear weapons program, although the evidence it used to make its case -- mainly the purchase of special aluminum tubes to make gas centrifuges -- was disputed by other intelligence agencies that believed the tubes were mainly for civilian use.

Staff writer Jonathan Weisman contributed to this report.