BAGHDAD, Iraq, Dec. 8 - A suicide bomber shoved aside a fare collector at Baghdad's main bus terminal on Thursday, forced himself onto a crowded bus and blew himself up, causing an enormous explosion that killed at least 30 people and reduced the bus to a charred, mangled husk, witnesses and police officials said.
Separately, an Islamic insurgent group claimed in an Internet posting to have executed a kidnapped American security consultant, according to the SITE Institute, which tracks jihadist Web sites. The posting neither named the consultant nor provided evidence to back the claim.
The group, the Islamic Army in Iraq, said on Tuesday that it had abducted Ronald Alan Schulz, 40, a native of North Dakota, and threatened to kill him within 72 hours unless all detainees in Iraq were released.
The United States Embassy in Baghdad confirmed Mr. Schulz's kidnapping, but a spokeswoman said on Thursday that the embassy was unable to verify the Internet claim.
The bus bombing occurred at about 10:45 a.m. in the Nahda terminal in central Baghdad, which serves as the station for buses bound for the Kurdish north and the Shiite-dominated south. The bus was full and about to leave the terminal when the bomber forced his way on, witnesses said.
"The fare collector saw the suicide bomber and told him that the bus was full," said Ahmad Adnan Khalil, 20, another fare collector who witnessed the incident. "So the suicide bomber pushed the collector and blew himself up."
Several bystanders said the blast was immediately preceded by a smaller explosion that appeared to come from the luggage hold, as if explosives had been hidden in a suitcase and detonated remotely. The explosions killed most of the passengers and several people at a nearby food stand, and wounded at least 25 others, the police said.
The bombing, the second large-scale suicide attack in Baghdad this week, appeared to have been driven by deadly sectarian intent, since the bus was filled with passengers headed toward the predominantly Shiite city of Nasiriya in the south.
The insurgency, in part led by disgruntled Sunnis, has sought to provoke sectarian discord and further divide the nation by attacking unprotected Shiite targets, including mosques and marketplaces.
Though buses have long been a favorite target of suicide bombers in Israel, Thursday's bombing in Baghdad may have been the first suicide bus bombing by Iraq's insurgency, Maj. Gen. Rick Lynch, a senior American military spokesman, said at a news conference on Thursday.
This was not the first attack at the Nahda terminal. A triple car-bomb attack at rush hour on Aug. 17 killed at least 43 people and wounded 89. After the August attack, however, neither the terminal's operators nor the Iraqi security authorities appeared to have adopted tighter security measures, like thorough checks of passengers, luggage or vehicles.
The American military command has warned of a surge in violence in advance of Dec. 15 elections for a full, four-year National Assembly. On Tuesday, two suicide bombs killed at least 36 police officers at the capital's main police academy and wounded at least 72 others, including an American contractor.
This week's bombings seemed to cast doubt on claims by American military commanders that they have sharply undermined the ability of the insurgents to carry out attacks. Since last spring, the American military has conducted more than a dozen sweeps of towns and villages along the Euphrates River in western Iraq in an effort to disrupt what many commanders call "a rat line" of foreign fighters, including suicide bombers, entering from Syria.
General Lynch estimated on Thursday that about 95 percent of suicide bombers are foreigners operating under Abu Musab al-Zarqawi, the Jordanian-born leader of Al Qaeda in Mesopotamia.
"His weapon of choice is the suicide bomb," General Lynch said of Mr. Zarqawi. "But his capability is significantly reduced."
While the number of suicide attacks has dropped to 23 in November from 52 in October, rebels continue to strike with devastating effect.
Several months ago, American officials pointed to another drop in suicide attacks - to 40 in August from 70 in May - to indicate the success of their offensives in western Iraq. The following month, however, the numbers began to climb again, to 49 in September and 52 in October.
The Islamic Army in Iraq, one of scores of militant groups that compose the loosely knit insurgency, said in its Internet posting that it had killed "the American security consultant," calling him "the American pig."
"We will provide you with pictures of killing him later," it said.
If the claim is true, it would be the first execution of an American kidnapping victim since Steven Vincent, a freelance journalist, was abducted on Aug. 2 and killed several hours later. His captors never issued any demands and remain at large. Two kidnapped Algerians were executed by their captors in Iraq in late July.
Mr. Schulz's abduction became publicly known on Tuesday when Al Jazeera, the Arab satellite network, broadcast a videotape of him wearing jeans and a dark jacket, with his hands tied behind his back.
That kidnapping was the latest in a wave of abductions of Westerners in Iraq in the past two weeks, including those of Susanne Osthoff, a German archaeologist taken on Nov. 25; four aid workers - an American, a Briton and two Canadians - kidnapped on Nov. 26; and Bernard Planche, a French engineer, abducted in Baghdad on Monday.
More than 200 foreigners, many of them from Arab nations, have been kidnapped since the American invasion in the spring of 2003. At least 50 have been killed and about 20 are still missing. Over the summer, insurgents began a campaign to attack and abduct foreign Arab envoys in an effort to drive their diplomatic missions out of Iraq.
President Bush told reporters on Tuesday that the United States would not submit to kidnappers' demands. In what seemed to be a reference to those remarks, the militant group's Internet posting on Thursday accused Mr. Bush of "arrogance" and said "he had an irresponsible response, so he is to be held accountable."
Mr. Schulz attended Jamestown High School in Jamestown, N. D., where was an avid hunter and fisherman, said Jim Nayes, who taught vocational agriculture at the school. Mr. Schulz eventually moved to Alaska and had been living there for about five years, Mr. Nayes said.
Also on Thursday, the American military command announced the death of two American servicemen. A soldier died when his convoy struck a hidden bomb in eastern Baghdad on Thursday morning, and a marine was killed on Wednesday when a bomb exploded next to his vehicle in Ramadi. Another soldier was killed near Habbaniya on Tuesday when his vehicle hit a mine, the military said.