Democratic Rep Kyrsten Sinema will win Arizona's US Senate race, defeating Republican Rep Martha McSally and flipping a seat that had been in GOP hands for 24 years.

McSally conceded the hard-fought race on Monday night -- six days after the election -- as county-by-county tallies of mailed-in ballots continued to increase Sinema's lead.

The concession by McSally means the Senate will have 51 Republicans and 47 Democrats in the upcoming Congress. The Senate race in Florida remains undecided and the Mississippi Senate race is going to a runoff later this month.

She will replace outgoing Senator Jeff Flake, a conservative Republican who is a critic of Trump.

Arizona is a border state with increasing numbers of Latino immigrants.

Democrats, who reclaimed control of the House of Representatives in the midterms, have now flipped two Senate seats: Arizona and Nevada.

Republicans gained Senate seats in Indiana, Missouri and North Dakota.