WASHINGTON (Reuters) -- The United States said explicitly for the first time on Thursday it would veto a Palestinian bid for full UN membership, drawing an immediate rebuff from the Palestinians who vowed to press ahead.

George Mitchell, a former US envoy, said he saw little chance of talking the Palestinians out of seeking full UN membership for a state in the Gaza Strip and West Bank, a step requiring a vote in the Security Council where the United States holds a veto.

Diplomats have said it is not clear what the Palestinians will do when the U.N. General Assembly opens on Sept. 19. They could seek lower status as a "non-member state," which would require a simple majority of the 193-nation assembly.

The United States and Israel have repeatedly argued against Palestinian moves at the United Nations, arguing the only way to solve the dispute is through negotiations and that a UN push will leave both sides even further from peace talks.

"The US opposes a move in New York by the Palestinians to try to establish a state that can only be achieved by negotiations," US State Department spokeswoman Victoria Nuland told reporters at her daily briefing.

"So, yes, if something comes to a vote in the UN Security Council, the US will veto."

Nuland's comments marked the first explicit veto threat by the United States, although U.S. officials have stressed for months their opposition to the Palestinians taking the issue to the United Nations.

In response, Nabil Abu Rdainah, a spokesman for Palestinian President Mahmoud Abbas, issued a statement saying: "We are going to the United Nations to request a full membership for Palestine in order to protect the rights of our Palestinian people and the concept of two-state solution."

Mitchell, the former US special envoy for Middle East peace, said on Thursday there was little chance US officials would be able to persuade Palestinian leaders not to seek greater recognition at the United Nations.

Mitchell, who stepped down in May after more than two years of fruitless efforts to make peace between the Palestinians and the Israelis, was downbeat about the odds of making progress in the coming months but more optimistic over the longer term.

David Hale, Mitchell's replacement as the US Middle East peace envoy, and White House aide Dennis Ross met Abbas on Wednesday in the latest US effort to halt the Palestinians' UN push.

"I think there was and is little likelihood that they will succeed in that effort," Mitchell said at a conference on peacemaking at Georgetown University in Washington.

Mitchell, who helped broker the agreement that ended the Northern Ireland conflict, earlier told the audience he saw little chance at the moment that Israeli and Palestinian leaders can take steps to overcome their impasse.

"In the short term, and I mean by that the next few months, it's difficult to be overly optimistic, to put it mildly," he said.

"But I believe that in the medium and longer term there is a basis for believing that they will be able to take those steps primarily because the current circumstance, in my judgment, is unsustainable and both societies face very large risks from a continuation of the conflict."