Employees of the Ilyich Iron and Steel Works hold a rally to protest against actions of armed separatists in Mariupol, eastern Ukraine on May 20, 2014. Factories sounded their sirens in two cities of eastern Ukraine on Tuesday and steel workers held a peace rally in support of a call by Ukraine's richest man, Rinat Akhmetov, for protests against armed separatists who planned to disrupt a May 25 presidential election. Reuters/Maxim Zmeyev

(Reuters) - Ukrainian forces surrounded the strategic rebel-held port city of Mariupol on Friday in a dawn attack launched as part of a broader military operation to reclaim control of eastern Ukraine.

The rebel forces, who oppose the pro-European leadership in the capital Kiev and want to be part of Russia, said five of their fighters had been killed in the battle for Mariupol, Ukraine's largest Azov Sea port.

Mariupol, which has changed hands several times in weeks of conflict, is strategically important because steel is exported through the port and the city lies on major roads from the southeastern border with Russia into the rest of Ukraine.

"The operation began at 4.50 a.m. (0150 GMT). It's proceeding according to plan and has been successful. All key strongholds held by the terrorists are being brought under control," Interior Minister Arsen Avakov said on Facebook.

"The area where the ​​operation is being carried out in the center of Mariupol has been cordoned off."

He said National Guard and Interior Ministry units were involved, as well as special armed forces.

One National Guard field commander said the rebels had destroyed an armored personnel carrier during the fighting. Another said four members of the National Guard had been wounded in the fighting but none had been killed.

President Petro Poroshenko intensified the military operation against the separatists after he was elected on May 25. The rebels took several cities and towns in east and southeast Ukraine after Russia annexed Crimea in March following the overthrow of Poroshenko's Moscow-leaning predecessor.