WASHINGTON, US - President Barack Obama, seeking to breathe new life into his drive to remake US health care, will address a rare joint session of the US Congress on September 9, congressional aides said Wednesday.

The speech will come one day after lawmakers return from a month-long August break, during which Obama's approval ratings slipped and doubts appeared to grow about his embattled efforts to extend medical care to more Americans.

The aides, who spoke on condition of anonymity, said Obama had accepted an invitation from Democratic House Speaker Nancy Pelosi and Senate Democratic Majority Leader Harry Reid to address Congress.

"Our nation is closer than ever to achieving health insurance reform that will lower costs, retain choice, improve quality and expand coverage. We are committed to reaching this goal," Pelosi and Reid wrote to the president.

"We would like to invite you to address a Joint Session of the Congress on Wednesday, September 9, 2009 on health insurance reform," they said in a letter made public by their offices.

News of the speech came amid Democratic complaints that Obama has not done enough to sell the country on his plan to overhaul health care despite broad support for changing the existing system.

Advocates of the plan say the existing US approach costs vastly more but underperforms when compared to other major industrialized countries and leaves some 46 million Americans uninsured.

Obama also faces pressure from core supporters who have warned him not to dilute or drop plans to offer government-run health care in order to get sweeping legislation through the Congress.

But Senate Republicans, and some swing-vote Democrats, have said they will oppose the overhaul if it includes the so-called "public option."

"I don't think the problem is the messaging, I think the problem is the substance," said Senate Republican Minority Leader Mitch McConnell.

Dropping the government plan, McConnell said, would be "a step in the right direction" but not enough to win his support.

A senior Obama administration official said that strident Republican attacks on the health care plan from senators like Chuck Grassley and Mike Enzi had clarified that the opposition party was not interested in framing a bipartisan approach.

The official said it was time to begin to pull together "various strands and solutions" from the four bills that have been marked up in congressional committees and other proposals.

"Basically all the cards are on the table," the official said.

The decision to address Congress was among a number of options that Obama was considering to advance the plan, and appeared to have been made on Wednesday, before the president left for his Camp David retreat.

House Minority Leader John Boehner said Republicans "want to hear what the president has to say," but underlined: "it's clear the American people don't want a new speech, they want a new plan."

Republicans also seized on news of the speech to tie Obama's efforts to former president Bill Clinton's failed first-term attempt to overhaul the US health care system, which included a similar September appeal to lawmakers.

"The last time a president addressed a joint session of Congress to sell a radical government health care overhaul was Bill Clinton on September 22, 1993," said Brad Dayspring, spokesman for Representative Eric Cantor, the number two Republican in the House of Representatives.

Clinton's speech drew mostly rave reviews, although Republicans immediately criticized what they described as a lack of specifics. The White House formally submitted its health care legislation two months later.

Health care legislation has cleared three House committees, and leaders of major Senate panels have said they hope to unveil a draft bill around mid-September.