BAGHDAD, Iraq -- Iran's foreign minister made a historic trip to Baghdad yesterday, pledging to secure his country's borders to stop militants from entering Iraq and saying the "situation would have been much worse" if Tehran were actually supporting the insurgency, as the U.S. has claimed.

Iranian envoy Kamal Kharrazi's trip -- two days after Secretary of State Condoleezza Rice paid a surprise visit to support the war-ravaged country's political process -- was the highest-level visit by an official from any of Iraq's six neighboring countries since the ouster of Iraqi President Saddam Hussein two years ago.

Kharrazi, who held talks with Iraqi Prime Minister Ibrahim Jaafari, President Jalal Talabani and Foreign Minister Hoshyar Zebari on a day of deepening sectarian violence, vowed that his country was committed to supporting Iraq's political and economic reconstruction and would do all it could to improve security conditions. "We believe securing the borders between the two countries means security to the Islamic Republic of Iran," Kharrazi said.

Zebari said militants have infiltrated from Iran into Iraq, "but we are not saying that they are approved by the Iranian government."

New British Defense Secretary John Reid also visited Iraq yesterday, traveling to Baghdad and Basra on his first foreign trip. The stream of visitors is aimed at shoring up the new Iraqi leadership, which is caught in a surge of violence that has killed more than 470 people since the government was announced April 28.

Ties between neighboring Iraq and Iran improved after the ouster of Saddam, who led an eight-year war against Iran during the 1980s in which more than 1 million people died.

The Iranian envoy's visit comes at a time of spiraling violence fueled by foreign extremists and rival groups of Sunnis and Shiites.

U.S. troops backed by helicopters battled scores of insurgents holed up in two houses in Mosul, 225 miles northwest of Baghdad.

Three Islamic clerics -- a Shiite and two Sunnis -- were shot and killed in Baghdad, police said yesterday, a day after Iraq's prime minister vowed to use an "iron fist" to end sectarian violence.

Another 17 Iraqis were killed yesterday: two Iraqi officials in separate Baghdad drive-by shootings, six truck drivers delivering supplies to U.S. forces north of the capital, a former member of Saddam's Baath Party and his three grown sons, three Mosul police officers and two soldiers in Baghdad.

A U.S. soldier was killed and a second was wounded when a roadside bomb struck their patrol near Tikrit, 80 miles north of Baghdad, the military said.