By Asif Shahzad, Ariba Shahid and Mubasher Bukhari

ISLAMABAD, Aug 24 (Reuters) - U.S. President Donald Trump spoke with Pakistan's army chief Asim Munir last week ahead of Munir's trip to Tehran on Monday, according to three Pakistani sources, with one saying the main U.S. request was to use Pakistan's influence to bring Iran back to negotiations.

Trump last week warned of economic consequences against any country that provided "any type of lifeline to Iran" as he seeks to isolate the Islamic Republic. Iran has in turn vowed ‌to shut down all oil exports from the Gulf.

Last week's call, which has not been previously reported,was initiated by Trump, according to two Pakistani sources.

The White House Office of the Press Secretary did not immediately respond to a request for comment. It was unclear what Munir and Trump discussed in detail.

Munir arrived in Tehran on Monday accompanied by Pakistan's Interior Minister Mohsin Naqvi, Pakistan's military confirmed, saying it was "part of Pakistan’s efforts to promote regional peace and stability."

Pakistan, which has played a key mediating role between the U.S. and Iran this year, has remained a trading partner of Iran throughout the recent fighting.

Munir was expected to meet with people close to Iran's Supreme Leader Ayatollah Mojtaba Khamenei on Monday, one Pakistani source said.

Another Pakistani government source said while U.S.-Iran tensions were a focus of the visit, Munir was also expected to discuss recent attacks by Iran-aligned Houthi fighters on Pakistan's ally Saudi Arabia as well as the recently signed mutual defence agreement between Pakistan, Turkey and Saudi Arabia.

Pakistan's earlier mediation efforts between the U.S. and Iran resulted in an interim peace agreement signed in June. But the Islamabad Memorandum quickly faltered, amid new fighting between Iran and the U.S. that has threatened to spiral into a wider regional conflict in recent weeks.

"There is a trust deficit on both sides," a third Pakistani source said about the U.S. and Iran, adding that Munir's aim was to mitigate the lack of mutual trust on Monday.

(Reporting by Asif Shahzad, Ariba Shahid and Mubasher Bukhari; Editing by Rick Noack and Hugh Lawson)