(Adds U.S. Senator Lieberman's news conference, background)

By Zeeshan Haider

ISLAMABAD, Jan 9 (Reuters) - Pakistan rejected on Wednesday remarks by the U.N. nuclear watchdog chief that Pakistan's nuclear arsenal could fall into the hands of Islamist militants, and allayed the fears of a U.S. senator visiting Islamabad.

Mohamed ElBaradei, head of the International Atomic Energy Agency (IAEA), expressed his fears about Pakistan's nuclear weapons in an interview with the pan-Arab daily Al-Hayat.

His comments were widely reported in Pakistani newspapers on Wednesday and echo concerns raised by some U.S. nuclear experts and politicians worried by the militant violence and political turmoil that is rocking the government of President Pervez Musharraf.

Pakistan's Foreign Ministry dismissed ElBaradei's remarks as "unwarranted and irresponsible".

"Pakistan rejects the statement by Dr ElBaradei," Foreign Ministry spokesman Mohammad Sadiq told a regular news briefing.

U.S. Senator Joseph Lieberman, visiting Islamabad separately, said he had been briefed by Khalid Kidwai, a retired general heading up the Strategic Plans Division, and came away convinced that the nuclear arsenal was securely held.

Pakistan is a key ally in the U.S.-led campaign against global terrorism but deteriorating security in the country, particularly after last month's assassination of opposition leader Benazir Bhutto, has raised international concern about the safety of its nuclear weapons.

"I fear that chaos ... or an extremist regime could take root in that country which has 30 to 40 warheads," Elbaradei was quoted as saying in the interview. Other estimates have put the number of warheads at 60.

Despite concerns, especially since the Sept. 11, 2001, attacks on the United States, U.S. military and defence officials say the weapons are safely under Pakistani control.

Sadiq said a three-member U.S. Congressional delegation visiting Pakistan this week had met officials of the military-led Strategic Plans Division, which has oversight for Pakistan's nuclear weapons. He gave no details.

Lieberman told a news conference in Islamabad, that General Kidwai had reassured him.

"Yes, he did allay my fears."

The senator, who chairs the Senate Committee on Homeland Security and Governmental Affairs, added that he would carry that message back to Congress.

The security of Pakistan's nuclear programme, begun in the early 1970s, has became a focus of greater international concern after A.Q. Khan, the head of the programme, confessed on national television in 2004 to selling nuclear know-how to Iran and Libya.

The next day he was pardoned by Musharraf. Despite the proliferation breach, the United States imposed no sanctions.

Pakistan has emerged as a hot topic in the United States as politicians compete in their parties' primaries in the run-up to the November presidential election.

Senator Hillary Clinton said that, if elected, she would propose to Musharraf that a U.S., and possibly British, team could be deployed to safeguard Pakistan's nuclear assets. (Additional reporting by Simon Cameron-Moore, Editing by Matthew Jones)

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