US President Donald Trump warned on Thursday that if Hamas continues to kill people in Gaza, "we will have no choice but to go in and kill them".

The threat - just three days after the Middle East agreement was signed - came amid reports that Hamas fighters used the truce to quickly and violently reassert control over the Gaza Strip, targeting Palestinians they considered to have collaborated with Israeli forces during the war.

"If Hamas continues to kill people in Gaza, which was not the deal, we will have no choice but to go in and kill them," Trump wrote in a post on Truth Social. "We appreciate the attention given to this matter."

Trump has so far played down the prospect that US forces would need to get involved in peacekeeping as negotiators try to move towards a permanent end to the war.

"We won't need the American army," Trump told reporters on Wednesday. "We will help Israel."

The President's warning, however, represented an abrupt change from his much more optimistic tone immediately after the ceasefire agreement, first announced last week. At one point, he declared that the war was effectively "over". Trump also played down early reports that Hamas was cracking down on so-called gangs in the Palestinian territory, even as footage of an apparent public execution was released. (CNN)