ISLAMABAD, Pakistan — Pressure mounted Sunday on President Asif Ali Zardari to investigate allegations that Pakistan’s ambassador to the United States had sought American help to rein in the Pakistani military, as opposition politicians called for action and records appearing to implicate the ambassador were leaked to the news media.

The ambassador, Husain Haqqani, returned to Islamabad, the Pakistani capital, on Sunday and met briefly with Mr. Zardari, according to an aide to the ambassador, who spoke on the condition of anonymity. The two were planning to meet again on Monday.

Mr. Haqqani has been accused of having a memo sent in May to Adm. Mike Mullen, then the chairman of the Joint Chiefs of Staff, asking the United States to help prevent a coup in Pakistan and in exchange offering help with the war on Islamic militants and an array of other diplomatic and security considerations.

Mr. Haqqani, who denies having anything to do with the memo, has offered to resign and was recalled by the government to explain his position.