BAGHDAD, Iraq – Iraq’s interim prime minister blamed the American led coalition yesterday for “great negligence” in the ambush that killed about 50 soldiers heading home after graduation from an American-run training course and warned of an escalation of terrorist attacks.

Underscoring the warning, insurgents made a new threat of nationwide attacks against American and Iraqi forces “with weapons and military tactics they have not experienced before” if American forces try to storm the terrorist stronghold of Fallujah.

Prime Minister Allawi told the Iraqi National Council, a government oversight body, that coalition forces’ negligent handling of security was responsible for Saturday’s deadly ambush along a highway near the Iranian border.

“It was a heinous crime where a group of National Guardsmen were targeted,” Mr. Allawi said. “There was great negligence on the part of some coalition forces.”

He said the Defense Ministry began an investigation into whether terrorists had infiltrated military ranks to obtain information about troop movements.

Mr. Allawi did not explain how the coalition had failed in its responsibilities to the Iraqi troops, who were traveling to southern Iraq in three buses after graduating from a training course in Kirkush. The buses had no armed escort and the soldiers were not carrying weapons.

However, in an interview with Al-Arabiya TV, the defense minister, Hazem Shaalan, blamed the recruits, who in their eagerness to get home decided to leave immediately after their graduation and take an unauthorized route.

“They are to blame. They graduated at 12 p.m. and could have delayed their trip,” he said. Mr. Shaalan added that neither the Defense Ministry, the Kirkush commanders, nor the American-run forces were to blame.

“They are the ones who chose this road that led them to this ugly result,” he said of the victims. “There might have been some people who gave information about them to hostile sides.”

Some of the bodies were found in rows – shot execution-style through the head – at a site about 95 miles east of Baghdad, the Defense Ministry said. Other bodies were found on a burned bus nearby. The American command did not respond directly to Mr. Allawi’s comments, but said in a statement: “This was a cold-blooded and systematic massacre by terrorists. They and no one else, must be held fully accountable for these heinous acts.”

In his appearance before the council, Mr. Allawi also warned that more terrorists were massing in Fallujah, a city west of Baghdad, and “you should expect an escalation in terrorist acts.”

Mr. Allawi has told Fallujah leaders that they must surrender extremists, chief among them Jordanian terrorist Abu Musab al-Zarqawi, or face attack. His comments yesterday appeared aimed at preparing Iraqis for the eventuality of such an attack, which could inflame public opinion in Iraq and elsewhere in the Arab world.

In a videotape obtained by Associated Press Television News, terrorists calling themselves the “factions of the Islamic Resistance Movement in Iraq” warned that if the Americans try to overrun Fallujah, “we swear in the name of God that all armed factions will attack all military and civilian targets of the occupation forces and the interim government.”

The warning was delivered by a masked gunman dressed in an old-style Iraqi army uniform, flanked by seven other men. The speaker accused the Iraqi government of “aborting a peaceful solution with the people of Fallujah.”

“We will attack them with weapons and military tactics they have not experienced before and in the ways and forms of our choosing,” he added.

Separately, a video posted yesterday on a terrorist Islamic Web site showed what it claimed was a Japanese captive kidnapped by Mr. al-Zarqawi’s group and threatened to behead him within 48 hours unless Japan pulls its troops from Iraq.

Prime Minister Koizumi rejected the demand.

“I won’t withdraw troops,” he was quoted as saying by Japan’s Kyodo news agency after receiving news of the hostage threat.

He also ordered Hiroyuki Hosoda, his chief cabinet secretary, to confirm the facts of the case and to consider measures to free the hostage if the kidnapping is confirmed, according to Kyodo.

The man, who had long hair and wore a white T-shirt, was identified only as someone connected to the Japanese armed forces. He spoke briefly in halting English and Japanese, addressing himself to Mr. Koizumi.

“They asked me why Japanese government broke the law and sent troops to Iraq,” the man said in English. “They want Japanese government and Koizumi prime minister, they want to withdraw the Japanese troops from Iraq or cut my head.”