The message has been relayed to coup leader General Sonthi Boon-yaratglin but Thailand remained tight-lipped, taking a wait-and-see approach at least until after the next straw vote among the 15-member UN Security Council (UNSC) in New York.

According to the source, Washington felt Surin was a strong candidate.

"He is an Asian, a moderate Muslim and a former foreign minister who is well known and respected in the international community," said the source, who spoke on condition of anonymity.

Washington believes China and the four other permanent members may also rate Surin as the best available choice for the position.

South Korea's candidate, Foreign Minister Ban Ki-moon, came first in the last two straw votes in the UNSC but Washington still thinks Surin is a better choice - because he is seen as a person better able to bridge the gap of misunderstanding between the West and the Muslim world.

Complicating the matter is the fact that Thailand already has a candidate - former foreign minister Surakiart Sathirathai, who has close ties with ousted premier Thaksin Shinawatra. Surakiart is currently in New York lobbying for the UN's top post.

Sonthi's decision to endorse Surakiart came as a surprise to many, considering the former foreign minister rushed to defend Thaksin last week. He was on CNN defending the PM as soldiers and tanks were taking over the capital.

Surakiart also has the endorsement of Asean. China, one of the UN Permanent-5 that has the power to veto any candidate, has "applauded" Asean's decision to support Surakiart. But at the same time, China also sent a letter to Sri Lanka informing them Beijing would take Colombo's candidacy seriously. The other Permanent-5 members are the US, Britain, France and Russia.

Moreover, Asean's support for Surakiart is questionable as other members are reportedly looking to field their own candidate if and when Surakiart - whose bid has been marred by controversy - withdraws from the race.

Another setback for Surakiart came after the coup, when China's ambassador to the UN told reporters: "I think definitely the situation in Thailand makes this issue more complicated."

The UN Permanent-5 is expected to go back to the drawing board after this weekend's straw vote. The last two straw votes among the 15 UN Security Council members had Surakiart coming a distant third.

But the source said: "There is a concern that Surakiart will not withdraw from the race so that another candidate from Asean can have a shot at the top post."

According to the UN Charter, "the Secretary General shall be appointed by the General Assembly upon the recommendation of the Security Council."

But in practice, the Council nominates only one candidate and then forwards the name to the UN General Assembly for endorsement. This makes the selection a "done deal" before it goes to the floor for endorsement.

According to a Bangkok-based diplomat from one of the Permanent-5 countries, selecting the next UN chief is an extremely difficult process to predict because it follows no formal procedure. The selected person is an outcome of diplomatic tussle and compromise between the five permanent members, he said.

In addition to Surakiart and Ban, other candidates include UN undersecretary-general for public affairs Shashi Tharoor of India; Jordan's UN Ambassador Prince Zeid al Hussein; former UN disarmament chief Jayantha Dhanapala of Sri Lanka; former Afghan Finance Minister Ashraf Ghani and Latvian President Vaira Vike-Freiberga, the only non-Asian.