Pakistan's ambassador to the United States has stepped down following claims he wrote a memo appealing for Washington's help in reining in Pakistan's powerful military.

Ambassador Hussain Haqqani resigned Tuesday following a meeting with Pakistani Prime Minister Yousuf Raza Gilani and army and intelligence officials in Islamabad. The prime minister's spokesman said Haqqani was asked to step down so that an investigation into the memo can be properly carried out.

Haqqani had been the focus of a political dispute over a letter sent in May to Admiral Mike Mullen, the top U.S. military official at the time. The memo called for U.S. help in preventing a military coup in Pakistan following the American raid that killed former al-Qaida leader Osama bin Laden.

The ambassador has denied writing the letter, which was reported sent to Mullen by Mansoor Ijaz, a U.S. citizen of Pakistani origin.

The ambassador is a close ally of Pakistani President Asif Ali Zardari.