BAGHDAD, Iraq — Efforts to form a unity government suffered a new setback Sunday when Iraqi leaders postponed a parliament session after failing to agree on a prime minister. Bombs targeted Shiites near a mosque and on a bus as attacks nationwide killed at least 35 people.

Four more Marines were reported killed in fighting west of Baghdad, bringing the U.S. death toll for this month to 47 — compared with 31 for all of March.

U.S. officials believe the best way to stem the violence is for the Iraqis to establish a government comprising Shiites, Sunnis and Kurds, paving the way for the United States to start withdrawing its 133,000 troops.

But progress has stalled over Sunni and Kurdish opposition to the Shiite choice of Prime Minister Ibrahim al-Jaafari to head the new government. With al-Jaafari refusing to step aside, acting speaker Adnan Pachachi called a parliament session for today, hoping the full legislature could agree on a new leadership after the politicians failed.

On the eve of the session, Pachachi announced a delay of "a few days" to give the religiously and ethnically based parties more time to agree on the new prime minister, president and five other top posts that require parliamentary approval.

Before the announcement, Shiite official Hussain al-Shahristani told Sunni and Kurdish leaders that his bloc, which controls 130 of the 275 parliament seats, would decide what to do about al-Jaafari "within the coming two days," Kurdish lawmaker Mahmoud Othman said.

Majority Shiites have been giving similar assurances for the past two weeks, and it was unclear how soon the issue could be settled. CNN quoted Pachachi as saying that with the parliament delay, it could take another month to form a government.

Voters chose the new parliament on Dec. 15, but the legislature met briefly only once last month.

The bitter fight over al-Jaafari has heightened friction among the rival parties, raising the specter of deadlock over other top jobs.

"This delay will affect everything," Sunni lawmaker Naseer al-Ani said.

Pressure has been mounting on the Shiites to replace al-Jaafari, whom critics accuse of failing to curb sectarian tension that has soared since the Feb. 22 bombing of a Shiite shrine in Samarra, which triggered a wave of reprisal attacks against Sunnis.

Four Marines — three from Regimental Combat Team Five and one from the 2/28 Brigade Combat Team — died Saturday in Anbar province, the U.S. command said Sunday.

Their deaths raised to at least 2,376 the number of U.S. military members who have died since the war began in March 2003, according to an Associated Press count. The rise in U.S. casualties followed a sharp drop in March, which saw the lowest number of American dead in Iraq since February 2004.