A pro-Russian rebel takes cover as a woman from inside a kiosk looks on, during what the rebels said was an anti-terrorist drill in DonetskBy Adrian Croft and Tulay Karadeniz ANTALYA, Turkey (Reuters) – U.S. Secretary of State John Kerry called for a halt to fighting around the Ukrainian coastal town of Shyrokyne on Wednesday as NATO backed his demand that Russia fully implement a Ukraine ceasefire agreement. Kiev fears separatists may try to seize the port to help cement a long-term hold on eastern districts of Ukraine. Russia denies providing any troops or arms to support rebellion in eastern Ukraine, which has killed more than 6,100 people, and accuses Kiev of violating the ceasefire. Kerry flew in to the Turkish seaside resort of Antalya to brief NATO foreign ministers on his eight hours of talks on Tuesday with Russian President Vladimir Putin and Foreign Minister Sergei Lavrov.