* Clear risk of alliances between Islamic State, locals

* Italy willing to take lead role within UN framework

* Boat arrivals up 60 pct so far this year

ROME, Feb 18 (Reuters) - Italy called on Wednesday for urgent international action to halt Libya's slide into chaos, and pledged it was ready to help monitor a ceasefire and train local armed forces.

The U.N. Security Council is due to meet later on Wednesday to discuss Libya, where two rival governments, each backed by former rebels who toppled Muammar Gaddafi in 2011, are battling for power.

The growing danger became apparent on Sunday when Islamic State released a video showing the beheading of 21 Egyptian Coptic Christians in Libya.

Foreign Minister Paolo Gentiloni told parliament that possible alliances between local militias and IS militants, inspired by counterparts in Syria and Iraq, risked destabilising neighbouring countries.

"The deterioration of the situation on the ground forces the international community to move more quickly before it's too late," he said in a special address on the crisis.

"There's a clear risk of alliances between Daesh and local groups," he said, using a common Arabic name for Islamic State. "The situation must be monitored with the maximum attention."

Italy, whose southern islands are only around 300 kilometres (186 miles) from the Libyan coast, has watched in alarm as the country has unravelled since Western forces helped topple Gaddafi.

Hundreds of thousands of migrants have arrived in southern Italy in unsafe boats, their departure from Libya facilitated by people smugglers operating freely in conditions of near-anarchy. Last week, more than 300 were reported to have died attempting the perilous crossing.

As well as fuelling anti-immigrant sentiment in Italy, which is suffering a deep economic slump of its own, the crisis has also heightened security fears, particularly after this week's beheadings of the Egyptian Christians.

Gentiloni spoke with U.S. Secretary of State John Kerry late on Tuesday when Italy joined the United States, France, Germany, Spain and Britain in calling for a national unity government in Libya.

He said Wednesday's Security Council meeting had to produce concrete signs that the scale of the crisis had been recognised, and he said Italy was ready to help monitoring a ceasefire and training a regular army within the framework of a U.N. mission.

He said the surge in migrant arrivals, up nearly 60 percent in the first six weeks of the year to more than 5,300, was clearly connected with the security situation in Libya.

Italy ended its "Mare Nostrum" search-and-rescue mission in October but the EU-backed Triton mission that replaced it has been criticised as inadequate.

"We must significantly reinforce Triton to bring it into line with the reality of the enormous scale of the problem," Gentiloni said. (Reporting by James Mackenzie and additional reporting by Steve Scherer; Editing by Raissa Kasolowsky)