Al Qaeda's No. 2 leader urged Afghans in a new videotape Thursday to rise up against U.S.-led forces in Afghanistan, prompting President Hamid Karzai to denounce the terror fugitive as "the enemy of the Afghan people." Four U.S. soldiers were killed in combat in eastern Afghanistan.

The posting of Ayman al-Zawahiri's videotape on an Islamic Web site followed a coalition military warning Wednesday that "significant violence" lies ahead in southern Afghanistan, where thousands of troops are fighting a deadly Taliban resurgence.

A U.S. official said the tape appeared to be authentic but a technical analysis was being performed.

The taped message was al-Zawahiri's sixth this year and was posted on a Web site known as a clearing house for Al Qaeda and other militants' statements. A U.S. intelligence official

"I am calling upon the Muslims in Kabul in particular and in all Afghanistan in general and for the sake of God to stand up in an honest stand in the face of the infidel forces that are invading Muslim lands," said al-Zawahiri, wearing a white turban and sitting in front of a black backdrop with an automatic rifle next to him.

The Egyptian-born fugitive also called on "the young men of Islam, in the universities and schools of Kabul, to carry out their duties in defense of their religion, honor, land and country."

The 3 1/2-minute tape, entitled "American Crimes in Kabul," appears to have been made the day after a May 29 accident in which a U.S. military truck crashed into traffic in Kabul, killing up to five people. The incident sparked anti-foreigner riots in Kabul that left about 20 people dead — the deadliest unrest in the Afghan capital since the fall of the Taliban in 2001.

Countrywatch:Afghanistan

"I direct my speech today to my Muslim brothers in Kabul who lived the bitter events yesterday and saw by their own eyes a new proof of the criminal acts of the American forces against the Afghani people," al-Zawahiri said on the videotape.

Unlike al-Zawahiri's previous messages, which appeared aimed at Americans, the latest video has no English subtitles. He spoke in Arabic, and Web sites carried translations in Pashtun and Farsi, two languages widely spoken in Afghanistan.

Asked about the new tape, Karzai blamed al-Zawahiri for Afghanistan's massive suffering before and after the Sept. 11, 2001, attacks.

"He is first the enemy of the Afghan people, and then the enemy of the rest of the world," Karzai said during a press conference. "He killed Afghans for years — thousands — and then he went to America and destroyed the twin towers."

"We in Afghanistan want him arrested and put before justice."

In renewed violence, four U.S. soldiers were killed and another wounded Wednesday while trying to block the movement of enemy forces in the eastern Nuristan province, the military announced in a statement Thursday.

Ground troops and attack planes were called in to continue the assault through the night, but it was unclear if there were any enemy casualties.

Afghan and coalition forces have been operating in eastern Afghanistan along the Pakistan border since mid-April targeting Al Qaeda and Taliban militants.

Late Tuesday, militants bombed two coalition convoys in southern Afghanistan, killing one civilian bystander and wounding 13, including six Canadian soldiers, the military said.

The attacks came as the U.S.-led coalition warned that major battles were likely to continue as Taliban fighters resist a large-scale military push in southern Afghanistan ahead of an imminent security handover to NATO-led forces.

"We are seeing the enemy operating in larger groups. They are fighting hard. They are clearly trying to stop our efforts to move into certain areas," coalition spokesman Col. Tom Collins said.

Coalition and Afghan forces launched Operation Mountain Thrust in earnest last week with more than 10,000 Afghan, British, Canadian and American troops deploying in the largest anti-Taliban offensive since the former regime's 2001 ouster.

More than 600 people, mostly militants, have been killed since May. At least 10 coalition soldiers also have been killed in combat in that time.

Al-Zawahiri and Al Qaeda leader Usama bin Laden were hosted by the Taliban before their ouster. They both are now believed to be hiding in the rugged border frontier between Afghanistan and Pakistan.

The new message by Al-Zawahiri is part of a dramatic increase in videos and audiotapes recently by Al Qaeda, including three by bin Laden.