Prospects for a gun control bill's passage in the US Senate in coming weeks got a boost today with a bipartisan agreement on background checks for gun buyers.

Democratic Senator Joe Manchin of West Virginia and Republican Senator Pat Toomey of Pennsylvania brokered the deal.

According to a Senate Democratic aide, the measure would expand criminal background checks for prospective gun buyers to include sales made at gun shows and online.

Sales among friends or family members would be exempt from the requirement, which is designed to close many loopholes.

Analysts have said as many as 40% of gun buyers have been able to avoid background checks under the current system.

The proposal for expanded background checks, which includes a requirement for sellers to keep records of sales, appears to be Mr Obama's best hope for meaningful gun control legislation in the wake of the December massacre of 20 children and six adults at a school in Newtown, Connecticut.

Several family members of victims from the shootings at Sandy Hook Elementary School in Newtown are in Washington this week to help Mr Obama's team pressure politicians for action.

More controversial parts of the president's plan, such as a ban on rapid-firing "assault" weapons such as the one used in Connecticut and limits on the capacity of ammunition magazines, appear to have a slim chance of clearing the US Senate, which is led by the Democrats.

Such measures face an even tougher road in the Republican-controlled House of Representatives.

Mr Manchin's participation in the deal on background checks is widely seen as boosting the prospects for the legislation's passage.

The senator represents a state where gun ownership has long been passionately protected and where attempts to regulate guns have been strongly opposed.

Tomorrow, the Senate is scheduled to hold its first test vote on a gun control bill.

More than a dozen conservative Republican senators have threatened a filibuster aimed at delaying consideration of any gun restrictions.

But other Republicans, many acknowledging public opinion polls that have shown that more than 80% of Americans favour expanded background checks, have said Mr Obama's proposals should get a floor vote in the Senate.

Senate Leader Harry Reid, the Nevada Democrat who schedules votes in the chamber, has echoed that position.