Hours before Israeli Prime Minister Benjamin Netanyahu embarked Sunday on a trip to Washington to mend U.S.-Israeli ties, tensions built in the West Bank when Israeli soldiers shot dead two Palestinian teenagers they say threatened them with a pitchfork and ax.

The shootings brought the Palestinian death toll to four during the last two days in the Nablus region. On Saturday, two Palestinian teens were shot by soldiers after a clash with Jewish settlers over a water well.

"This escalation by Israel and the killing of Palestinians every day is going to sabotage efforts by the U.S. and others to restart peace talks," said Nabil abu Rudaineh, a spokesman for Palestinian Authority President Mahmoud Abbas.

Sunday's violence erupted after an Israeli military patrol stopped to question two Palestinians they deemed suspicious, Israeli military officials said.

"When they got out of their vehicle, the Pal- estinians tried to stab a soldier with a pitchfork and an ax," an army spokeswoman said.

Palestinian officials said the two were farmhands, both 17, who were shot as they sat on the ground in military custody.

Meanwhile, human rights groups challenged Israeli army assertions that the two teenagers at the well Saturday were shot with rubber bullets.

Mohamed Qadus, 16, died Saturday of chest wounds. Asaud Qadus, 19, believed to be a cousin, died Sunday of a shot to the head, hospital officials said.

Officials with B'Tselem and the Popular Struggle Coordination Committee, two groups that work on behalf of Palestinians, released photos of the teenagers' injuries that they said show the army used live rounds. The photos included an X-ray image of a bullet they said was lodged in one teen's skull and an entrance and exit wound from the other's chest.

"There's no way rubber bullets could cause that kind of damage," said Jonathan Pollak, a spokesman for the Popular Struggle Coordination Committee.

Israeli officials insisted that only rubber bullets were used, but said they are investigating the case.

On Thursday, a rocket fired into southern Israel from the Gaza Strip killed a Thai worker in the farming community of Netiv Haasara, just north of Gaza. Israel retaliated with two airstrikes against militant Palestinian targets in Gaza, wounding about 15 people.

The weekend violence came as Netanyahu left for Washington to attend a conference of a pro-Israel lobbying group and to meet with Obama administration officials after this month's high-profile diplomatic scuffle over contentious Israeli housing construction.

During a Cabinet meeting Sunday, the prime minister indicated that he was not prepared to heed a U.S. request to halt the construction on land in Jerusalem that Israel seized after the 1967 Middle East War.

"Our policy toward Jerusalem is the same policy of all Israeli governments in the past 42 years and it has not changed," Netanyahu said. "From our point of view, construction in Jerusalem is like construction in Tel Aviv. These are the things which we have made very clear to the American administration."

U.S. officials had been pushing for a halt to a 1,600-unit East Jerusalem project that was approved during Vice President Joe Biden's recent trip, triggering a diplomatic flap between the two countries.

Netanyahu has been more receptive to other U.S. demands, including making such goodwill gestures to Palestinians as releasing prisoners or relaxing border restrictions in Gaza.

U.N. Secretary-General Ban Ki-moon, who visited Gaza on Sunday, called upon Israel to allow more humanitarian and construction supplies into the area.