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 the challenges that lie ahead include ending the bloodshed in war-torn Syria.

Israeli Prime Minister Benjamin Netanyahu said the U.S.-Israeli strategic alliance 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 President Vladimir Putin sent a telegram to Mr. Obama with his congratulations, a spokesman said, adding that the Kremlin leader received the U.S. election news “very positively.”

British Prime Minister David Cameron, who is visiting Jordan, tweeted his “warm congratulations” to Mr. Obama and said he looks forward to continuing cooperation with him. Mr. Cameron said the “appalling” events taking place in Syria make clear that Britain and the United States “must do more to try [to] solve this crisis.”

In Kenya, home of Mr. Obama's father, both national leaders praised the president's re-election. President Mwai Kibaki said it is confirmation of the American people's confidence in their leader. Prime Minister Raila Odinga said the election result will rekindle faith in democracy worldwide, and especially in Africa.

Mr. Obama's paternal grandmother lives in the Kenyan village of Kogelo, where she and other villagers celebrated the election's outcome. She told reporters she believes her grandson was re-elected because he has the capacity to love other people. And, as any grandmother would ask of her grandson, she implored Mr. Obama to do his best at his job.