By Michelle Nichols

UNITED NATIONS (Reuters) - Turkey told the United Nations on Friday that it had started conducting air strikes in Syria against Islamic State militants because the Syrian government was neither capable or willing to tackle the radical Islamist group.

Turkey has faced increasing cross-border violence along its 900-km (560-mile) frontier with Syria, where Islamic State militants have taken advantage of a more than four-year long civil war to seize swathes of territory.

In a letter to U.N. Secretary-General Ban Ki-moon and the U.N. Security Council, Turkey cited Article 51 of the U.N. Charter, which covers an individual or collective right to self-defense against armed attack, as justification for its action.

"It is apparent that the regime in Syria is neither capable of nor willing to prevent these threats emanating from its territory which clearly imperil the security of Turkey and safety of its nationals," wrote Turkey's Deputy U.N. Ambassador Levent Eler in the letter, seen by Reuters.

"Syria has become a safe haven for (Islamic State). This area is used by (Islamic State) for training, planning, financing and carrying out attacks across borders," he wrote.

Turkish warplanes attacked Islamic State targets in Syria for the first time on Friday, joining a U.S.-led coalition that has been bombing Islamic State targets in Syria for the past 10 months.

Eler said Turkey "initiated necessary and proportionate military actions against (Islamic State) in Syria, including in coordination with individual members of the Global Coalition,

in order to counter the terrorist threat and to safeguard its territory and citizens."

Under Article 51 of the U.N. Charter, the 15-member Security Council must immediately be informed of any action that states take in self-defense against armed attack.

After long being a reluctant partner in the U.S.-led coalition against Islamic State, Turkey has also approved the use of its air bases by U.S. and coalition aircraft to mount strikes against Islamic State.

A Syrian government crackdown on a pro-democracy movement in 2011 sparked a civil war. The United Nations has said that some 220,000 people have been killed and an estimated 7.6 million are internally displaced. Another 4 million people have fled Syria.

(Reporting by Michelle Nichols; Editing by Bernard Orr)