BEIJING, May 15 (Reuter) - China conducted an underground nuclear test on Monday, a Foreign Ministry spokesman said.

He gave no details of the time, place or nature of the test, the Xinhua news agency said.

Earlier, the Australian Seismological Centre in Canberra said it had detected a presumed underground nuclear explosion on Monday in western China.

The centre said in a statement the explosion took place at 0405 GMT on Monday and estimated the blast at 40 to 150 kilotonnes of TNT.

The underground blast is thought to have been detonated at China's Lop Nor test site inXinjiang province and registered about 5.9 on the Richter scale, the centre said.

"It looks like a Lop Nor test. They are pretty distinctive," seismologist Spiro Spiliopoulos told Reuters.

The blast would have been felt at seismological stations around the world and was the first nuclear explosion detected at Lop Nor since last October.

"It's the same general pattern and looks quite similar to explosions we have had there in the past," said Spiliopoulos, adding that Monday's blast was similar in size to the October test.

Xinhua quoted the Foreign Ministry spokesman as saying that China will cease nuclear testing once a comprehensive test ban treaty comes into effect.

"On the question of nuclear tests, China has always exercised great restraint. It stands for the complete prohibition and thorough destruction of nuclear weapons and has taken an active part in the Geneva talks on the comprehensive test ban treaty," the spokesman said.

China has made it clear on many occasions that it will cease nuclear testing upon the entry into force of the comprehensive test ban treaty, he stated.

"China strongly calls upon other nuclear-weapon states to respond positively to China's proposal, expeditiously conclude, through negotiations, a treaty on the non-first-use of nuclear weapons against each other and reach an agreement on not to use or threaten to use nuclear weapons against non-nuclear-weapon states and nuclear-free-zones," he said.