The three Israeli teenagers who were kidnapped and murdered in the occupied West Bank last month were shot at least 10 times with a silenced gun, a US official in the investigation has said.

The official said that the killings appeared to be premeditated, contradicting some Israeli and Palestinian speculation that the perpetrators intended to use the teens for a prisoner exchange but panicked and shot them.

"There were 10 gunshots," said the official, who spoke to Reuters on condition of anonymity.

The use of a silencer on the weapon to kill the three teenagers had led US investigators to believe that the kidnappers had plotted to kill the youths from the beginning of the incident, the official added.

The three teenagers - 16-year-olds Naftali Frenkel and Gilad Shaar and 19-year-old Eyal Yifrach - disappeared while hitchhiking home at night from the Jewish settlement of Gush Etzion.

Their bodies were discovered near the village of Halhul, north of Hebron, where two suspects in the case - Marwan Qawasmeh and Amer Abu Aisha - lived.

Israeli police believe that the teens' deaths were the motivation behind far-right Jews kidnapping and burning Palestinian teenager Mohammed Abu Khdeir in a suspected revenge killing in Jerusalem last week.

The manhunt for the suspects is continuing after Israel launched a wide-ranging operation in the West Bank to find them. More than 400 people, most said to be members of Hamas, were arrested and more than 1,300 sites were searched.