Bernie Sanders, the US senator who is challenging Hillary Clinton for the Democratic presidential nomination, has won nominating contests in Alaska and Washington.

The Associated Press called the race in Washington in Mr Sanders' favour with 31% of the vote in, he was leading Mrs. Clinton by 76% to 23.7%.

The Vermont senator still faces an uphill battle to upset the Clinton campaign, but the victories could dampen calls for him to wrap up his campaign in the interests of party unity.

"We are making significant inroads in Secretary Clinton's lead and ... we have a path to victory," Sanders told cheering supporters in Madison, Wisconsin.

"It is hard for anybody to deny that our campaign has the momentum."

But Mr Sanders would need to win up to two-thirds of the remaining delegates to catch Clinton, who will keep piling up delegates even when she loses under a Democratic Party system that awards them proportionally in all states.

Jim Manley, a Democratic strategist and Clinton supporter, said the wins were a boost for the Sanders campaign, but added Clinton had "too big a lead" for it to make a real difference.

"These wins will help him raise more funds for the next few weeks but I don't think it changes the overall equation," he said.

All three contests on Saturday were caucuses, a format that has favored Sanders because it requires more commitment from voters.

They also were in states with fewer of the black and Hispanic voters who have helped fuel Clinton's lead.