The US and a top UN official have separately called for an immediate investigation into the deaths of two apparently unarmed Palestinian youths killed last week by Israeli border policemen in the West Bank.

The young men were shot dead on Thursday as Israeli police responded to demonstrations marking the 66th anniversary of the Nakba - or "catastrophe".

However, Israeli Foreign Minister Avigdor Lieberman brushed off demands from at home and abroad for an investigation.

The pressure came after video footage emerged appearing to show the two Palestinians being shot unprovoked, prompting calls from Washington and the United Nations for a full investigation.

"I reject any demand" for an international investigation, Lieberman told reporters on a tour of the West Bank settlement of Ariel.

"Such an incident will be investigated regardless of any demand," he remarked, denouncing world criticism of the incident as "hypocrisy."

Closed-circuit video footage released by Defence For Children International and B'Tselem appeared to show separate incidents in which the two youths were shot as they walked down the same street near Ramallah.

Although clashes were taking place in the area on that day there is no visible evidence of ongoing unrest in the footage. 

But the Israeli army immediately dismissed the footage as having been "edited," and said it was investigating the incident.

An Israeli commentator criticised official attempts to downplay the incident, joining the calls for an investigation.

"We live in a world that is tainted by hypocrisy, double standards, changing values and a flexible code of behaviour... that is the situation," wrote Ben Caspit of Israel's daily Maariv newspaper on Thursday.

"The correct Israeli response is to comply with the American call issued yesterday, and to launch an immediate, in-depth, true and quick investigation. And most importantly, transparent. We must not have any secrets."

State Department spokeswoman Jen Psaki earlier said the United States had been in contact with the Israeli government.

"We are closely following this incident and the video. We're seeking additional information from the government of Israel," she said in Washington.

"We look to the government of Israel to conduct a prompt and transparent investigation to determine the facts surrounding this incident, including whether or not the use of force was proportional to the threat posed by the demonstrators."

She added: "We are encouraging the government of Israel to conduct their own investigation."

Assistant UN secretary general for political affairs Oscar Fernandez-Taranco also urged a probe.

"It is of serious concern that initial information appears to indicate that the two Palestinians killed were both unarmed and appeared to pose no direct threat," said Fernandez-Taranco.

"The UN calls for an independent and transparent investigation by the Israeli authorities into the two deaths, and urges Israel to ensure that its security forces strictly adhere to the basic principles on the use of force and firearms by law enforcement officials," he said at a briefing of the UN Security Council.

Israel has said the border police had tried to quell a violent demonstration by about 150 Palestinians.

But Palestinian leaders have said the slain boys were unarmed and posed no threat, accusing Israel of using "excessive and indiscriminate violence."