President Hamid Karzai was warned of a weekend assassination plot against him, Afghanistan's intelligence chief said Tuesday, acknowledging that failings by the security services allowed militants to launch the attack.

Meanwhile, a homicide attack killed 18 people, including 11 police, in an eastern province, officials said. Thirty-six people, including two Australian journalists, were wounded.

Amrullah Saleh told Parliament the plot to kill Karzai was hatched last month and the gunmen had rented the hotel room they opened fire from 45 days before the attack.

Karzai and other dignitaries escaped unharmed from Sunday's assault during a ceremony in Kabul marking Afghanistan's victory over the Soviet occupation of the country in the 1980s. Three other people, including a lawmaker, died.

Three of the attackers also were killed in a gunbattle with security forces after the assault, Karzai's government said, but the Taliban said three other insurgents got away.

"We had technical information ... that this work would happen," Saleh told a National Assembly session broadcast live on national television. "We passed this information to the national security (adviser) and to the president of Afghanistan."

Despite stringent measures by security services to protect the event, "the result is that we failed," Saleh said.

An Afghan intelligence official has said about 100 people were rounded up for questioning after the attack. Some of those questioned have since been freed, officials say. The official spoke on condition of anonymity because he was not authorized to speak to the media.

"Tragically, the attackers succeeded in getting close enough to fire some shots," said a statement issued by U.S. Ambassador William Wood.

It took authorities two minutes to defeat the attack, Wood said.

Saleh, Defense Minister Abdur Rahim Wardak and Interior Minister Zarar Ahmad Moqbel were summoned to explain to lawmakers what happened Sunday.

All three lost no-confidence votes against them by lawmakers on Tuesday, but not by a high enough margin to press for their ouster.

Daud Sultanzoy, a lawmaker, demanded all three security officials resign — although there was no immediate sign that would happen.

Several members of police, the intelligence service and members of the president's security detail were being questioned for negligence, while a police officer has been arrested on suspicion of involvement in the attack, Saleh said.

The attack in the Afghan capital underscored the fragile grip of Karzai's government in the face of Taliban insurgents.

Secretary of State Condoleezza Rice on Monday said the attack showed Karzai's administration is under a strong threat.

Afghanistan has "determined enemies who will do anything to disrupt the democratic progress that the Afghan people have made," Rice said.

Sunday's lapse brought questions about the readiness of Karzai's government to follow up on its demand for Afghan police and the army to take greater control of security. U.S. and NATO-led troops provide security in much of the country now.

But the White House said it was unfair to criticize Afghan security forces because insurgents had been able to stage an attack.

"When it comes to dealing with terrorists like the Taliban or Al Qaeda, they just have to have even ... a little bit of an impact for everyone to say that they had a big victory," White House press secretary Dana Perino said.

The attack was sure to bring a sense of unease in Kabul, which has been spared the worst of the violence as fighting escalated between the Taliban and international troops.

In the volatile east of the country, a homicide attack Tuesday targeting counter-narcotics police killed 18 people, including 11 police, and wounded 36 other people, the Interior Ministry said in a statement. The Taliban claimed responsibility for the attack.

The blast also wounded two Australian journalists, according to a spokeswoman for Australia's Department of Foreign Affairs and Trade. They were not identified.

The attack occurred in the Khogyani district of Nangahar province. Abdul Mohammad, a provincial police official, said the bomb went off in front of the office of the district chief, who was among those hurt.

Maj. Martin O'Donnell, a spokesman for the NATO-led International Security Assistance Force (ISAF) in Afghanistan, said it had troops in the area of the attack, but there were no alliance casualties.

The militants opened up with small arms fire and rocket-propelled grenades before a homicide bomber blew himself up in a crowd that was taking cover, he said.

Violence has intensified since the Islamist militia's ouster from power in a U.S.-led invasion in 2001, killing a record 8,000 people last year, according to the U.N. More than 1,000 people, mostly militants, have died in insurgency-related violence so far this year, according to an Associated Press tally of figures from Afghan and Western officials.