World leaders are sending in their congratulations to U.S. President Barack Obama, who won a second term Tuesday in a tightly contested race with Republican challenger Mitt Romney.

U.N. Secretary-General Ban Ki-moon said he “warmly congratulates” Mr. Obama and looks forward to continuing to work together “in the spirit of the enduring partnership between the United States and the United Nations.” He noted some of the challenges that lie ahead — including ending the bloodshed in war-torn Syria.

Israeli Prime Minister Benjamin Netanyahu said that the strategic alliance between Israel and the United States is stronger than ever and that he will continue to work with President Obama. Mr. Netanyahu is a longtime friend of Mr. Romney, the president's challenger.

Russian Prime Minister Vladimir Putin sent a telegram to Mr. Obama with his congratulations, while Mr. Putin's spokesman said Mr. Putin received the news “very positively.”

British Prime Minister David Cameron tweeted his “warm congratulations” publicly and said he looks forward to continued work together. But in a visit to Jordan Wednesday, Mr. Cameron told reporters that Mr. Obama needs to do more to help end the Syrian civil war.

Also sending their congratulations were Australian Prime Minster Julia Gillard and leaders from China and Japan.