Prime Minister Tony Blair and other British leaders paid tribute Wednesday to the Belfast sisters who are campaigning against the IRA killers of their brother, a case being highlighted this week in Washington.

"Of course, we all pay tribute to the courage of the McCartney family and no one has made their case better than them, frankly," said Blair, referring to the five sisters of Robert McCartney (search), 33, who was stabbed and beaten to death outside a Belfast pub on Jan. 30.

He was attacked by a group that included well-known IRA (search) men from their Catholic neighborhood of Belfast in a fight that began inside the crowded pub.

Police say witnesses are unwilling to identify McCartney's attackers, who allegedly cleaned up forensic evidence and warned their audience they could face an IRA death sentence if they talked to police.

The sisters began a scheduled series of meetings Wednesday in Washington with the U.S. envoy to Northern Ireland, Mitchell Reiss (search).

They also were expected to meet U.S. Senators Edward Kennedy (search) and Hillary Clinton (search) and, on Thursday, will be guests at the White House for annual St. Patrick's Day festivities.

The sisters have repeatedly accused the IRA and its allied Sinn Fein (search) party of trying to minimize their movement's role in the killing and its cover-up.

The Sinn Fein-IRA movement initially denied any involvement and criticized police efforts to find McCartney's killers. But the McCartney sisters' campaign has forced a gradual turnaround. The IRA has expelled three members while Sinn Fein has suspended seven members who were in the pub when the attack on McCartney began.

But joint Sinn Fein-IRA hostility to the Northern Ireland police has proved a major stumbling block to the case. While witnesses have given statements to their own lawyers, they have refused to talk to detectives. The IRA instead told the sisters it was willing to shoot four people it blames for killing McCartney, an offer that provoked widespread outrage.

Sinn Fein has also admitted that two of its election candidates were in the pub when the attack began, but both witnesses claim they didn't see anything.

Blair made his pro-McCartney comments during a parliamentary debate with Michael Howard (search), leader of Britain's major opposition Conservative Party.

Howard said the sisters were demonstrating "extraordinary courage and tenacity" and had "exposed the evil of IRA thuggery." He said Sinn Fein, the major Catholic-backed party in Northern Ireland, was "linked to terror and organized crime and refuses to cooperate with the police."

Blair agreed, and reiterated that Sinn Fein would not take part in any new administration in Northern Ireland until the IRA fully disarmed and ceased all activities, including crime — Britain's position since October 2002.

Northern Ireland's Good Friday accord (search) of 1998 envisioned a future where the territory's British Protestant and Irish Catholic leaders governed together. But a power-sharing coalition that included Sinn Fein fell apart in 2002 amid chronic arguments over IRA activities, and a deal to revive the arrangement failed in December when the IRA refused to renounce crime or permit photos of its disarmament.

Blair said his government had made it repeatedly clear to Sinn Fein "that the price of going into government is giving up criminal activity and paramilitarism completely."