WASHINGTON - US Republican senators on Tuesday decided not to vote on their latest Obamacare repeal bill after it was clear their latest effort would fail.

"We made the decision since we don't have the votes, we're going to postpone it," said Senator Bill Cassidy after the Republican lawmakers met at their weekly luncheon.

"We haven't given up on changing the American health care system," Senate Majority Leader Mitch McConnell said. "We are not going to be able to do that this week, but it still lies ahead of us, and we haven't given up on that."

The last-gasp drive was aimed to kill off the Affordable Care Act, known as Obamacare, dismantling its insurance subsidy program and Medicaid expansion and convert their funding into block grants to states.

Three Republican senators have announced their opposition to the repeal proposal, ensuring that Republican senate leaders would be short of the votes they needed.

Among them, Rand Paul said the repeal bill would have left too much of Obamacare in place. John McCain said he would be voting no in large part because of a process he said was rushed and excluded Democrats.

Republican Senator Susan Collins from the northeastern US state of Maine announced her opposition to the repeal bill on Monday, citing its cuts to Medicaid and its lack of protection for people with pre-existing conditions.

In response, Democrats in the Senate urged their Republican colleagues to resume bipartisan negotiations to stabilize health insurance markets under the Obamacare.

Republicans hold a slim 52-48 majority in the Senate and must pass the bill with a simple majority before Sept 30 deadline.