BAGHDAD - A top American official expressed confidence Tuesday the United States and Iraq will finalize a long-term security pact on time next month despite strong opposition from Iran and a storm of criticism from Iraqi legislators who must ratify the deal.

David Satterfield, the State Department's top adviser on Iraq, said both sides were committed to reaching an agreement, which would also provide a legal basis for keeping U.S. troops here after the United Nations mandate expires at the end of the year.

"We're confident it can be achieved, and by the end of July deadline,'' Satterfield said of the agreement.

Satterfield bristled at suggestions by another senior U.S. official that it was "very possible'' Washington may have to extend the existing UN mandate.

"It's doable, that's where our focus is, not on alternatives,'' Satterfield told reporters. "We're focused on plan A because we believe plan A can succeed. ... We think it's an achievable goal.''

Stakes for both sides are high. An agreement would ensure long-term U.S. political and military support for Iraq and could help ease concerns that the country would fall under Iranian domination if U.S. troops leave.

The agreement also could serve to counter the spread of Iranian influence both in Iraq and the wider Middle East.

However, the deal is politically explosive in a country where many people are weary of the American military presence, considered an affront to Iraqi national pride.

U.S. officials believe much of the criticism has been orchestrated by Iran through militant Shiite groups including Hezbollah in Lebanon as well as anti-American cleric Muqtada al-Sadr, who is believed to be in Iran. Al-Sadr has called for weekly protests against the deal.

On Monday, Iranian supreme leader Ayatollah Ali Khamenei told visiting Iraqi Prime Minister Nouri al-Maliki that the presence of American forces was the "main obstacle'' blocking Iraq's "progress and prosperity.''

He told al-Maliki that Iraqis must "think of a solution to free'' the country from American troops rather than seeking a way to extend their stay.

Iran fears that if long-term U.S. military bases are established on Iraqi soil, the country could be used as a launching pad for attacks on the neighboring country.

Also Tuesday, the chief of Saddam Hussein's tribal clan was killed by a bomb glued to the undercarriage of his car, Iraqi police said.

Sheik Ali al-Nida, 65, was the leader of the al-Bu Nasir tribe, a large Sunni Arab clan of about 20,000 members, including Saddam's family.

One bodyguard was killed and three were wounded when the vehicle exploded as they drove through the Wadi Shishain area of Tikrit, a mostly Sunni Arab city about 130 kilometres north of Baghdad.

Al-Nida received Saddam's body after his 2006 execution and arranged the former dictator's funeral. In 2007, he founded a so-called Awakening Council in Saddam's home village of Ouja, partnering with U.S. forces to fight Sunni militants in the area.

Members of Saddam's tribe have been targeted before, but it was unclear whether it was because of their ties to the former Iraqi dictator or because of long-standing tribal rivalries.