WASHINGTON (AFP) - President Donald Trump's Republican Party won another seat in the US Senate on Wednesday (Nov 11), with a victory in Alaska that puts it one vote away from a majority in the upper house of Congress.

Incumbent Dan Sullivan was easily reelected with more than 57 per cent of the vote, according to CNN and NBC television projections.

His victory confirms the strong performance of the Republican Party in the congressional elections, which were held on Nov 3 at the same time as the presidential election won by Democrat Joe Biden.

The Republicans now have 50 seats, compared to 48 for the Democrats in the 100-seat Senate. Two seats are still to be filled in runoffs scheduled for Jan 5 in the southern state of Georgia.

The Democrats would have to win both seats to catch up with the Republicans and give Biden more room to implement his policies. It would then be Vice-President-elect Kamala Harris' voice that would decide the tie in a 50-50 vote in her role as president of the Senate.

No law can be passed in the United States without the upper house, which also has the power to approve the president's appointments: his secretaries, his ambassadors, and the judges, especially on the Supreme Court.

If the Senate remains Republican, Biden, who sat in it for 36 years, would have to use his skills of bi-partisanship and negotiation.

The President-elect said on Tuesday that he was confident that he would be able to work with a sufficient number of congressional lawmakers from the other side.

Trump has now won Alaska and three more electoral votes, US media said on Wednesday, in a boost that does not change Biden's White House win.

Biden now has at least 279 of the 538 electoral college votes - he needed 270 - and Trump now has 217.