US Secretary of State Colin Powell has urged Israel and the Palestinians not to allow the latest violence to derail the search for peace in the Middle East.
The gunmen approached the checkpoint dressed as soldiers
He was speaking on a day of bloodshed, when five Israelis and five Palestinians died in the first major clashes since last week's US-led peace summit in Jordan.
"We all have to work together to get this terrorism under control but, at the same time, not let terrorism stop us from moving forward," Mr Powell said in a television interview.
Early on Sunday, three Palestinian gunmen attacked the Erez army checkpoint between the Gaza Strip and Israel, killing four Israeli soldiers and wounding four others before themselves being shot dead.
Later in the day, another Israeli was killed along with two Palestinian gunmen in the West Bank city of Hebron, the Israeli army said.
The killings happened just hours after Palestinian militants vowed to maintain their campaign of violence against Israel.
Three groups took joint responsibility for the first attack, in what is being seen as a serious blow to Palestinian Prime Minister Mahmoud Abbas - also known as Abu Mazen - who pledged at the summit to crack down on militants.
Help for Palestinians
"What we have to do now is make sure we don't allow this tragic, terrible incident to derail the momentum of the [Middle East peace plan, known as the] roadmap that got started... last week," Mr Powell said.
ROADMAP MAIN POINTS
Phase 1 (to May 2003): End to Palestinian violence; Palestinian political reform; Israeli withdrawal and freeze on settlement expansion; Palestinian elections
Phase 2: (June-Dec 2003) Creation of an independent Palestinian state; international conference and international monitoring of compliance with roadmap
Phase 3 (2004-2005): Second international conference; permanent status agreement and end of conflict; agreement on final borders, Jerusalem, refugees and settlements; Arab states to agree to peace deals with Israel
"It is a difficult situation, but if it was an easy situation, it would have been solved many, many years ago," Mr Powell said.
The US reaction was measured, says the BBC's Steven Kingstone in Washington.
In the past, the US has blamed Palestinian leader Yasser Arafat and reasserted Israel's right to self-defence, our correspondent adds.
The softer tone reflects American confidence in Abu Mazen who - unlike Yasser Arafat - is regarded by Washington as someone to do business with, he says.
Mr Powell said the United States would give Abu Mazen practical help to deal with the militant groups.
"He needs to have his police forces rebuilt. He needs communications. He needs vehicles. He needs a lot."
France has also made a plea for restraint, saying acceptance of the roadmap was a sign that both sides desired peace.
A BBC correspondent in Jerusalem, Richard Galpin, says the Gaza attack - claimed by Hamas, the Islamic Jihad, and the Al-Aqsa Brigades - leaves the new Palestinian prime minister in "an extremely difficult position".
Israeli Government spokesman Avi Pazner said Mr Abbas "had to deal with the issue of terrorism immediately and not wait a single day" in the wake of the attack.
"We don't expect to see 100% results immediately, but if he does not fight the terrorists, we will," he said.
Following the attack Abu Mazen postponed a planned trip to Gaza aimed at pushing for a ceasefire.
One of the groups claiming responsibility for the attacks, the al-Aqsa Brigades, is linked to Yasser Arafat's Fatah movement, of which Abu Mazen is himself a long-standing member.
Israel has made it clear that the peace process cannot advance while such attacks continue.