Pakistan appears to expand nuclear site - report

* Think-tank says new building early in its construction

* Pakistan "determined" to make more weapons-grade plutonium

VIENNA, Feb 10 (Reuters) - Pakistan appears to be building a fourth military nuclear reactor, signalling its determination to produce more plutonium for atomic weapons, a U.S.-based think-tank said.

The report came as India and Pakistan agreed to resume peace talks that were broken off by New Delhi after the 2008 Mumbai attacks, a move that should help ease tensions in the volatile region.

The nuclear-armed neighbours have been under pressure from the United States to reduce tension because their rivalry spills over into Afghanistan, complicating peace efforts there.

The Institute for Science and International Security (ISIS), a think-tank specialising in nuclear proliferation issues, said it had obtained commercial satellite images from mid-January.

They showed "what appears to be a fourth reactor under construction at Pakistan's Khushab nuclear site," ISIS experts David Albright and Paul Brannan said in the Feb. 9 report.

"Pakistan is determined to produce considerably more plutonium for nuclear weapons," they wrote.