Several media reports have quoted unnamed US officials as saying the target was a suspected Syrian nuclear site being built with North Korean help.
"We are not playing 'trust me' in this negotiation. We are doing things in a very verifiable way"
Christopher Hill, chief US negotiator
Hill said he could not discuss allegations of North Korea's role in Syria's nuclear programme at an open hearing because that information was classified.
Instead he said he had "raised this issue" with North Korea officials, and he remained convinced that the North would stand by the deal.
"It is my professional judgment that by the end of this year, we will have clarity on their uranium enrichment such that we can be assured that a highly enriched uranium programme is no longer a threat to our country," he said.
Hill said the deal North Korea agreed to in February in talks with China, Japan, Russia, South Korea and the US was designed to prevent Pyongyang from getting rewards before compliance.
"We are not playing 'trust me' in this negotiation. We are doing things in a very verifiable way," he said.
|North Korea has agreed to shut its |
nuclear programme in return for aid
But representatives from both the Republican and Democrat parties expressed scepticism over the deal, questioning whether the Bush administration's rush to score a foreign policy success had been the driving force behind the deal.
"The administration is somewhat desperate for a foreign policy success," Brad Sherman, a Democrat said, mentioning what he called failed efforts in Iraq and Iran.
Meanwhile Republican Ed Royce said the United States "should be cracking down on this regime's illegal activity… not easing up".
Boyce said he was worried "that this process has us putting up carrots and putting down sticks."
In the latest round of six-nation talks, the North pledged to disable its main nuclear facilities and declare all its nuclear programmes by year's end in return for economic aid and political concessions.
Among the concessions offered by the US is the removal from of North Korea from a list of states that sponsor terrorism.
"We need to know if they're dealing honourably with us"
North Korea is known to the have supplied missile technology to Syria but has strongly denied accusations that it spreads its nuclear expertise beyond its borders.
Syria meanwhile has also denied receiving any North Korean nuclear help or embarking on any nuclear program.
But the cloud of secrecy over the Israeli air raid, and the stony silence from US officials has raised many questions.
Speaking at the House hearing on Thursday, Republican Dan Burton said it was important to know if North Korea was keeping to its word on the deal in general and on nuclear proliferation in particular.
"We need to know if they're dealing honourably with us," he said.
He added that if the White House wanted cooperation on paying for aid shipments to North Korea and other matters, it should tell lawmakers what it knows.
"You guys over there at state can't keep this from Congress. You've got to tell us what's going on," Burton said addressing Hill, who is a US assistant secretary of state.