President Barack Obama has said the pace of Israeli settlements in the Middle East has "gotten so substantial" that it has become harder to imagine an "effective Palestinian state", as he defended US' decision to abstain from voting at the UN Security Council against Israel.

Obama said the increase in settlement building will have "long-term consequences" for peace and security in the region.

"We are reaching a tipping where the pace of settlements, during the course of my presidency has gotten so substantial that it's getting harder and harder to imagine an effective, contiguous Palestinian state," Obama told CBS News on Sunday.

"I think it would have long-term consequences for peace and security in the region, and the United States, because of our investment in the region, and because we care so deeply about Israel, I think has a legitimate interest in saying to a friend, 'This is a problem.' And we've said it – look, it's not as if we haven't been saying it from Day One," he said.

"We've been saying it for eight years now. It's just that nothing seemed to get a lot of attention," Obama said, responding to questions on the recent US abstention from a UN Security Council vote on Israel.

He asserted that the US' decision to abstain from the vote last month did not cause a "major rupture" in the bilateral ties.

"I don't think it caused a major rupture in relations between the United States and Israel. If you're saying that Prime Minister Netanyahu got fired up, he's been fired up repeatedly during the course of my presidency, around the Iran deal and around our consistent objection to settlements."

"So that part of it wasn't new. And despite all the noise and hullabaloo – military cooperation, intelligence cooperation, all of that has continued," he said.

"We have defended them consistently in every imaginable way. But I also believe that both for our national interests and Israel's national interests that allowing an ongoing conflict between Israelis and Palestinians that could get worse and worse over time is a problem.

"And that settlements contribute. They're not the sole reason for it, but they're a contributing factor to the inability to solve that problem," Obama said.

In his final interview, Obama also defended his use of "red line" word against Syria's use of chemical weapons.

"I've got to tell you, though, I don't regret at all saying that if I saw Bashar al-Assad using chemical weapons on his people that that would change my assessments in terms of what we were or were not willing to do in Syria," he said.

"I think it was important for me as president of the United States to send a message that in fact there is something different about chemical weapons. And, regardless of how it ended up playing, I think – in the Beltway, what is true is Assad got rid of his chemical weapons," Obama said.

"And the reason he got rid of them is because – well, look

if 90 per cent or 95 per cent of those – chemical stockpiles were eliminated, that's a lot of chemical weapons that are not right now in the hands of ISIL or Nusra or, for that matter, the regime," Obama said.