Washington - Donald Trump was able to unilaterally reject a global climate pact, but to get action on his domestic priorities he must now wrestle with a divided US Congress under the shadow of an FBI Russia probe.

Lawmakers return to Washington on Monday under pressure to advance Trump's agenda, notably bills to repeal and replace Barack Obama's health care reforms and overhaul US tax law.

Trump is expected to pile on additional initiatives this week, including a $1 trillion plan for upgrading crumbling US bridges, highways and airports, and a proposal for privatizing the US air traffic control system. But despite Republican majorities in both houses, Congress has so far proved resistant to his ideas.

And with crises swirling around Trump - including a bombshell report that Trump son-in-law Jared Kushner sought a secret communications link to Russia - the Trump administration is desperate for a rallying win. There is a shrinking window for success; Congress has just seven work weeks before its members head home in late July for a summer recess that lasts until early September.

White House officials have privately expressed frustration with the slow pace on Capitol Hill, as well as some embarrassment about how an initial effort to ram health care changes through the House of Representatives collapsed in March.

A revised Obamacare repeal bill passed the House in May, and Senate Republicans are crafting their own version. But even Majority Leader Mitch McConnell has acknowledged it will be difficult to pass it through the Senate, where Republicans have a narrow 52-48 majority.

"Hopefully Republican Senators, good people all, can quickly get together and pass a new (repeal & replace) HEALTHCARE bill," Trump tweeted Wednesday, in a more-than-gentle nudge. But despite weeks of closed-door meetings, Republican senators have yet to unveil a new health bill.

The effort took a crushing blow last month, when a nonpartisan congressional study estimated the new House plan would leave 23 million more Americans uninsured than under current law.

Still, number two Senate Republican John Cornyn assured that an Obamacare repeal will be ready by the summer recess.

"We'll get it done by the end of July at the latest," he told the Chad Hasty radio show. Trump meanwhile signaled Thursday that tax reform is "moving along in Congress" and doing "very well."

"The president keeps saying the tax bill is moving through Congress. It doesn't exist!" House Democratic leader Nancy Pelosi retorted on Friday.

She said lawmakers only have the White House's bare-bones, one-page tax plan that outlines dramatic tax cuts for businesses and individuals.

Tax reform, repealing Obamacare, and immigration reform - including funding for Trump's wall on the US-Mexico border - were all part of his opening 100-day plan. That marker came and went April 29, with none of those targets met.

Even if Republicans are ready to jump headfirst into health care and taxes, distractions abound.

A special counsel is investigating Russia's US election meddling, and whether Trump's campaign coordinated or colluded with Moscow in that effort. Congressional committees are busy issuing subpoenas.

And in what could be an explosive moment, sacked FBI director James Comey testifies Thursday in an open Senate Intelligence Committee hearing.

Lawmakers will press him, in the open hearing and in another behind closed doors, on whether Trump urged Comey to stop investigating his fired national security adviser Michael Flynn.

Complicating the schedule, Congress must formulate a budget and vote on a spending bill before the fiscal year ends September 30. Republican divisions over spending priorities could make that task difficult.

In another squeeze, the administration is now urging Congress to vote to raise the debt ceiling before summer recess, due to weaker-than-expected government revenues.

Some Republicans oppose allowing government to borrow more money without commitments to reduce overall federal spending.

Acting soon would force lawmakers to take a controversial vote before they return to their districts.

If Trump's health care and tax priorities drag into late 2017 or beyond, they risk butting up against 2018's midterm election campaign, when Republicans, already saddled by Trump scandals, will be eager to avoid further controversy.