China's rejection on a proposed visit by the United States defense secretary, which shocked the Pentagon on Tuesday, is aimed at Washington's reluctance to adjust its tone on its disputed arms sale to Taiwan, Chinese experts said.

It has been five months since Beijing announced it would curtail military contacts, including high-level visits with the US, after the Obama administration notified the US Congress of a $6.4-billion arms sale to Taiwan.

"The US has not made any concrete moves to clarify its stance on the arms sales, and neither has shown its attitude. That's the reason Beijing doesn't want to thaw military ties," said Peng Guangqian, a Beijing-based military strategist.

US Defense Secretary Robert Gates has said publicly that he wanted to put military cooperation between the US and China back on track by such a visit after finishing an annual security conference in Singapore that started on Thursday.

But a senior US official said on Tuesday on condition of anonymity that Beijing rejected the proposed fence-mending visit, citing it was "not a convenient time".

"It certainly wouldn't be beyond the realm to speculate that this is sort of a continued ... reaction to the Taiwan arms sale," said the official.

At Thursday's meeting in Singapore for regional defence ministers, Gates said he believes China's military leaders were "significantly less interested" in building ties with Washington than the political leadership, according to Reuters.?

He also said he was disappointed the PLA leadership did not see that close military ties could help prevent security miscalculations.

Hong Kong-based Phoenix TV commented on Thursday that China's reluctance to receive Gates is possibly related to concerns that the visit could trigger suspicion from Pyongyang and destroy cooperation on the investigation into the sinking of a Republic of Korea warship.

But both experts dismissed such possibility.

"It's just speculation. I can't see apparent relations between the two cases," Peng said.

Jin Canrong, associate dean of the school of international relations at the Beijing-based Renmin University of China, said though some US generals visited Beijing on the sidelines of the Strategic and Economic Dialogue last month, "a visit by the US defense secretary marks official thawing of military ties, and it's not that easy".

Pentagon press secretary Geoff Morrell has complained about Beijing's "pattern" of freezing military contacts in retaliation for US arms sales to Taiwan.

"He (Gates) just doesn't believe that a relationship of this importance can take place in fits and starts. There needs to be a continuous, high-level engagement between these two powers and it can't be derailed by bumps in the road that will inevitably come up," Morrell said.

China also suspended military exchanges with the US in 2007 as the Bush administration planned to launch a multi-billion dollar arms deal with Taiwan.

But military relations are always the most sensitive parts of relations among large countries, said Jin.

"It is the easiest part to break and the most time-consuming to recover. That's normal."

Though Morrell said China needs to be more interested in meeting, Jin said Washington is actually the one more desperate for regular military exchanges.

"Among the strategic military powers the US listed around the world, the People's Liberation Army is the only one that they know little about, and it's also the fastest-changing one."