The appointment of an American ambassador to Singapore has been delayed again.

Ms K.T. McFarland, a former deputy national security adviser, has asked for her nomination to be withdrawn.

Her confirmation process had dragged on for months as senators questioned her role in the Russian issue that has dogged the Trump administration.

Ms McFarland, 66, is the second ambassador-designate within a week to drop out of a key post.

On Tuesday, Dr Victor Cha, a Georgetown University professor and senior adviser at the Centre for Strategic and International Studies, was dropped as the ambassador-designate to South Korea. He had apparently disagreed with the White House's approach to the Korean peninsula.

In a brief statement on Friday, President Donald Trump said: "I am disappointed that K.T. McFarland has withdrawn from consideration to be Ambassador to Singapore.

" K.T. served my administration with distinction. Unfortunately, some Democrats chose to play politics rather than move forward with a qualified nominee for a critically important post."

Singapore is currently Asean chair. The United States also does not have an ambassador to Asean yet.

In Washington circles, no alternative names have emerged yet as potential nominees for the Singapore post. Background checks and the confirmation process normally take several weeks or months.

Ms McFarland's confirmation process ran into controversy last year when it emerged that she had not revealed all that she knew about her former boss Michael Flynn's contacts with Russia's former ambassador to Washington Sergey Kislyak.

Mr Flynn, one of Mr Trump's biggest supporters during his campaign, was the President's first national security adviser. But he was fired after he was found to have misled Vice-President Mike Pence on the contact he had with Mr Kislyak, apparently to discuss the Obama administration's sanctions on Russia.

Ms McFarland's nomination was held up as it was sent back to the White House.

The administration nominated her again last month, even as analysts saw her confirmation as a long shot, given the animosity over the Russia issue.

Ms McFarland worked with former secretary of state Henry Kissinger, and former secretary of defence Caspar Weinberger during the Ronald Reagan administration. She was also a Fox News commentator on security issues.

Said Mr Trump: "I wish K.T. the best as she uses her considerable wisdom and skill as a commentator to explain to the American people how to make American foreign policy great again."

Ms Farland was known to have the ear of the President and was thought by some Washington insiders to be ambitious and planning to serve out her Singapore assignment and return to Washington as national security adviser.

Though she is the second ambassador-designate after Dr Cha to withdraw, there are differences between the two cases.

Ms McFarland's withdrawal was not over policy but her potential role in the Russia investigation, said vice-president Rafael Frankel of BowerGroupAsia.

"The Victor Cha case is more deeply concerning because of what he has publicly written about - the growing bellicosity of the Trump administration (on North Korea)," he told The Sunday Times.

Still, the delay in confirming ambassadors to the key positions is seen as a drawback for the US, especially in Asia where it is dealing with tension on the Korean peninsula and the South China Sea.