Pakistan's ambassador to Washington returned to Islamabad on Sunday to explain himself over claims that he wrote a letter seeking US help against the country's powerful military.

Hussain Haqqani, a close aide of President Asif Ali Zardari, has played a key role in helping Pakistan's civilian government navigate turbulent relations with Washington that nosedived over the US raid that killed Osama bin Laden.

Local media reports implicated Haqqani in a memo allegedly sent from Zardari to Admiral Mike Mullen, then America's top military officer, seeking to curtail Pakistan's military shortly after it was humiliated by the bin Laden killing.

Zardari reportedly feared that the military might seize power in a bid to limit the hugely damaging fallout in Pakistan after Navy SEALs killed bin Laden in the garrison city of Abbottabad on May 2.

The alleged memo, released last month by American businessman Mansoor Ijaz, said that a "new national security team" in Pakistan -- with US support -- could end ties between Pakistani intelligence and Islamist militants.

A senior government official told AFP that Haqqani "arrived early Sunday and is due to attend several meetings including a meeting with the president to explain the situation".

Haqqani has offered to resign over the row, but has denied any involvement with the document.

Pakistan's opposition leader Nawaz Sharif on Saturday demanded an independent commission to investigate the issue.

Defence minister Ahmed Mukhtar told a press conference in the eastern city of Lahore on Sunday that any decision on Haqqani's future would lie with Prime Minister Yousuf Raza Gilani and must be made after an investigation.

"If our ambassador is involved, then it is a matter related to the prime minister," Mukhtar said. "The prime minister just cannot dismiss Haqqani without giving him a charge-sheet and conducting an investigation."

Asked whether there was any chance of a military coup in the country, Mukhtar said: "I don't think so."

He added that a BlackBerry messenger conversation reported to have taken place between Ijaz and Haqqani was also under investigation, after it was leaked to Pakistani media this week in an apparent bid to prove the ambassador's involvement with the memo.