U.N. weapons inspectors, in a recent routine search of Iraqi missiles, discovered traces of the deadly VX nerve gas on warheads, a senior Clinton administration official disclosed Tuesday.

The discovery, to be detailed at the United Nations, will strengthen the hand of the United States in maintaining tough economic sanctions on Iraq when the issue is taken up by the Security Council on Thursday, said the official, insisting on anonymity.In New York, meanwhile, well-informed diplomatic sources also confirmed that Iraq had placed the nerve gas in its warheads, a development in missile power that Iraq consistently claimed it had failed to accomplish.

Those sources said Richard Butler, chief of the U.N. inspection team, reported to the council last week that Iraq had been told about "the preliminary results of the chemical analysis of certain excavated remnants of special war-heads."

"The Iraqi side rejected these results," the sources said. "Both sides agreed to continue further discussions on this issue. While informed of the commission's concerns, Iraq refused to undertake additional steps to clarify the extent of its attempts to produce the chemical warfare agent VX."

In Baghdad, Deputy Prime Minister Tariq Aziz said Iraqi scientists had experimented with VX but were unable to turn it into a weapon. He said in a letter to the Security Council that 1.7 tons of the agent had been produced but it was not of weapons grade, according to the official Iraqi News Agency.

The Washington Post on Tuesday reported the discovery of nerve gas fragments in missile warheads. Sources confirmed the finding to the Associated Press.

State Department spokesman James P. Rubin said, "It appears to be another case of UNSCOM (the U.N. special commission) having overcome Iraq's deceit on what it has done and it is doing."