WASHINGTON (AFP) - The United States on Wednesday condemned the killing of a Palestinian youth in apparent revenge for the murders of three Israeli teenagers, warning acts of vengeance would worsen an explosive situation.

"There are no words to convey adequately our condolences to the Palestinian people," Secretary of State John Kerry said in a statement.

President Barack Obama's national security advisor Susan Rice said on Twitter that the murder of 16-year-old Mohammed Abu Khder was "heinous" and called on both Israelis and Palestinians to avoid a cycle of retribution and revenge.

Khder was seen being forced into a car by three Israelis in east Jerusalem. Police later confirmed a body was found in a forest in west Jerusalem.

DNA tests proved the body was that of the missing teenager, his family said. In his statement, Kerry condemned the killing in "the strongest possible terms."

He said it was sickening to think a young boy could be "snatched off the streets and his life stolen from him and his family."

Kerry, who spoke by phone with Benjamin Netanyahu on Wednesday, noted that the Israeli prime minister had condemned the killing and urged Israelis not to take the law into their own hands.

"Those who undertake acts of vengeance only destabilize an already explosive and emotional situation," Kerry said.

"We look to both the government of Israel and the Palestinian Authority to take all necessary steps to prevent acts of violence and bring their perpetrators to justice.

"At this tense and dangerous moment, all parties must do everything in their power to protect the innocent and act with reasonableness and restraint, not recrimination and retribution." The attack is widely believed to have been carried out in revenge for the kidnap and killing of three Israeli teenagers by Palestinian militants.

Netanyahu told Kerry he had "asked for an investigation to find out who was responsible for the despicable murder as soon as possible, that anyone who takes a life will be punished severely," State Department spokeswoman Jen Psaki said.

The Israeli leader also said "that he had publicly called for all sides not to take the law into their own hands," she added.

Tensions have soared since June 12 when the three Israeli teenagers disappeared while hitchhiking in the West Bank. Their bodies were found on Monday, with Israel blaming Hamas and vowing to hit it hard.