Prospects for US Senate ratification of a landmark nuclear treaty with Russia this year dimmed Tuesday as a top lawmaker said he doubted the pact could be approved before 2011.

"This is a very complicated process. It cannot be done overnight," the chamber's number-two Republican, Jon Kyl, said when asked about fate of the new Strategic Arms Reduction Treaty (START). "There's still a lot to be done."

But US Director of National Intelligence James Clapper, speaking after a Senate hearing on US government employee security clearances, reiterated his support for the accord and said he hoped it would be passed soon.

"I think the earlier, the sooner, the better. You know, my thing is: From an intelligence perspective only, are we better off with it or without it? We're better off with it," Clapper told AFP.

Kyl said Democratic Senate Majority Leader Harry Reid had telephoned him Monday to ask whether Republican concerns about the treaty had diminished enough for the chamber to take it up in a year-end "lame duck" session.

"I replied I did not think so given the combination of other work Congress must do and the complex and unresolved issues related to START and modernization" of the US nuclear arsenal, Kyl said in a statement.

But State Department spokesman Philip Crowley said "it remains the Obama administration's belief that the ... New START treaty is in our national interest. And we believe it should be voted upon in this lame-duck session."

The administration "believes that we've answered all of their questions," he said.

"We've addressed their concerns, including concerns that Senator Kyl and others have expressed about ensuring that there is an effective modernization program as a companion to the New START treaty," Crowley added.

A senior State Department official, speaking to reporters on the condition of anonymity, expressed "disappointment" with Kyl's comments and said Clinton would discuss START and other business when she meets senators on Wednesday.

The START treaty -- signed by Russian President Dmitry Medvedev and US President Barack Obama in Prague in April -- restricts each nation to a maximum of 1,550 deployed warheads, a cut of about 30 percent from a limit set in 2002.

But Republicans have said they need to be sure that the US nuclear arsenal will be modernized to remain a credible deterrent and that the treaty will not hamper US missile defense efforts.

US Senate ratification requires 67 votes out of 100.

Republicans will have 47 votes when the next Congress convenes in January, up from 41, after routing Obama's Democratic allies in November 2 elections.