The Kerch Strait, which connects the Black Sea with the Sea of Azov, has become a flashpoint between Moscow and the West after Russia seized three Ukrainian ships and 24 sailors in November. Ukraine has accused Moscow of military aggression, while Russia claims the boats illegally crossed its border near Crimea.

NATO plans to expand its surveillance in the Black Sea to ensure that Ukrainian ships travel safely through an area effectively controlled by Russia, a top U.S. official in the Western military alliance said Tuesday.

The U.S. plans to offer a package to NATO defense ministers this week that “beefs up surveillance — both air surveillance as well as more of the NATO country ships going into the Black Sea” in support of Ukraine.

“It is a package... to assure that there’s a safe passage for Ukrainian vessels through the Kerch Strait [to] the Sea of Azov,” said Kay Bailey Hutchison, the U.S. ambassador to NATO.

NATO foreign ministers are holding their regular April meeting in Washington on Wednesday and Thursday as they celebrate the 70th anniversary of the U.S.-led alliance.

An unnamed senior U.S. official has said the NATO ministers will focus on the “defense and deterrence posture” in the Black Sea region. The official, speaking to reporters on condition of anonymity to preview the NATO meeting, said the ministers will seek to agree on a package of measures to bolster the military alliance's presence in the Black Sea.

Reuters contributed reporting to this article.