US President Barack Obama has said a partial government shutdown is due to an "ideological crusade" against his healthcare programme.

He blamed Republicans for the first shutdown in 17 years, which potentially put up to one million workers on unpaid leave.

National parks have been shut and medical research projects are stalled.

President Obama urged politicians to vote to keep government operations running without conditions.

"They've shut down the government over an ideological crusade to deny affordable health insurance to millions of Americans," he said in remarks in the White House Rose Garden.

"Many Representatives have made it clear that had they been allowed by Speaker (John) Boehner to take a simple up or down vote on keeping government open with no strings attached, enough votes from both parties would have kept the American people's government open and operating," he said.

The White House this evening turned down a proposal by House Republicans to pass a series of targeted funding bills, saying the piecemeal approach to funding the government was "not serious".

The proposal "shows the utter lack of seriousness that we're seeing from Republicans", White House spokesman Jay Carney said.

"If they want to open the government, they should open the government, and then we can negotiate about how we fund our budget priorities in the future," Mr Carney told reporters.

Earlier, federal agencies were directed to cut back services after politicians could not break a stalemate over an emergency spending bill.

"This shutdown was completely preventable. It should not have happened," President Obama wrote in a letter to government employees.

After House Republicans floated a late offer to break the deadlock, Senate Majority Leader Harry Reid rejected the idea.

He said Democrats would not enter into formal negotiations on spending "with a gun to our head" in the form of government shutdowns.

The political dysfunction at the Capitol also raised fresh concerns about whether Congress can meet a crucial mid-October deadline to raise the government's $16.7 trillion (€12.4 trillion) debt ceiling.

Both parties tried to deflect responsibility for the shutdown.

Mr Obama accused Republicans of being too beholden to Tea Party conservatives in the House of Representatives and said the shutdown could threaten the economic recovery.

If Congress can agree to a new funding bill soon, which Mr Obama insists not be tied to the reforms, the shutdown would last days rather than weeks, with relatively little impact on the world's largest economy.

But no signs of compromise emerged immediately as the Democratic-controlled Senate formally rejected an offer by House Republicans to break the logjam.

The political stakes are particularly high for Republicans, who are trying to regain control of the Senate next year.

Polls show they are more likely to be blamed for the shutdown, as they were during the last shutdown in 1996.

Spending for essential functions related to national security and public safety will continue, including pay for US military troops.