JERUSALEM — A fresh round of violence has threatened the diplomatic progress made at two Middle East summits last week, posing a challenge to the U.S.-backed "road map" peace plan.

The most deadly attack came Wednesday afternoon, when a Palestinian suicide bomber killed at least 16 people in a bus bombing in downtown Jerusalem.

The attack on Jaffa Road, near the Mahane Yehuda market, also wounded around 70 people.

Israel responded Wednesday with a helicopter strike in the Gaza Strip, reportedly killing at least four people.

The suicide bombing and Israel's retaliation were just the latest incidents in a bloody week.

At least five Israelis were killed and five wounded in separate incidents in the Gaza Strip and West Bank on Sunday.

Israel responded to Sunday's incidents on Tuesday, when it launched a helicopter strike in Gaza City that killed two bystanders but only wounded its intended target, senior Hamas official Abdel Aziz Rantissi.

Later Tuesday, Israeli helicopters attacked a residential era in the Gaza Strip, killing at least three Palestinians and wounding at least 30.

That came a day after Prime Minister Ariel Sharon began fulfilling Israel's obligations under the peace plan, removing some 10 illegal outposts in the West Bank, most of them unpopulated.

On Tuesday, Hamas vowed revenge for the assassination attempt — and on Wednesday, it claimed responsibility for the suicide bombing.

President Bush criticized the strike, saying it "does not contribute to the security of Israel."

Palestinian Authority officials also blasted the attack, but Palestinian Authority Prime Minister Mahmoud Abbas — who called it terrorism — said it would not halt progress on the road map peace plan.

Two dozen people were wounded in addition to Rantissi. Israeli government sources defended the attack, saying Israel would continue to fight terrorism as long as the Palestinian Authority fails to do so, Israel Radio reported.

Israel believes Rantissi played a vital role in organizing Sunday's attack in the Gaza Strip that killed four soldiers.

Earlier in the week, U.S. officials had urged Israel and the Palestinians not to let extremists set the diplomatic agenda, but this week's events showed the difficulties in following up on the steps taken at the summits in Egypt and Jordan.

In the Gaza Strip, four soldiers were killed and four were wounded when three gunmen dressed in Israeli army uniforms infiltrated an army position near the Erez Crossing.

The gunmen were killed in the ensuing exchange of fire.

An inquiry showed that the three attackers had slipped into a line of Palestinian workers entering the Erez industrial zone. Under cover of heavy fog, they crossed a fence and wall separating the industrial zone from the nearby army position.

After reaching the entrance to the outpost, they opened fire and threw grenades at a soldier standing outside, killing him. They then shot and killed two soldiers on guard duty at the outpost gates.

The gunmen then entered the outpost and opened fire on the soldiers, killing one and wounding four.

Soldiers in the base opened fire, killing the three. Duct tape and plastic handcuffs were found on the attackers' bodies, raising the possibility that they had intended to kidnap soldiers as bargaining chips for the release of Palestinian prisoners, Army Radio reported.

In other violence Sunday, an Israeli was killed when two gunmen opened fire near the Tomb of the Patriarchs in Hebron. Israeli troops killed at least one of the attackers. Earlier in the day, a border police officer was lightly wounded by a Palestinian gunman at a nearby checkpoint.

Three groups claimed joint responsibility for the Gaza attack — Hamas, Islamic Jihad and the al-Aksa Brigade, which is linked to the Fatah faction of Palestinian Authority President Yasser Arafat.

The attack was seen as a direct challenge to Abbas, who angered some groups by his conciliatory remarks at the summit with Bush and Sharon in Aqaba last week.

Accusing Abbas of conceding on Palestinian demands, Hamas froze cease-fire contacts with the Palestinian Authority and renewed its vow to carry out terror attacks against Israelis.

Israel, meanwhile, called on the Palestinians to immediately take steps to crack down on terrorism.

U.S. officials urged the sides not to let extremists undermine the opportunity for peace. Secretary of State Colin Powell and National Security Adviser Condoleezza Rice said such actions by opponents of the process were expected, but the United States remained committed to advancing the road map.

"This is the time when both leaders have to do everything they can" to move forward on the road map, Powell told "Fox News Sunday."