Libyan militias battle for airport

Libya's Tripoli International Airport, pictured here in 2012, has been controlled by brigades since 2011.

• NEW: National airline announces airport will be closed for three days


• A weak central government and rivalry among militias fuel the Libya crisis


• Brigades from Zintan have controlled the airport since 2011


• Armed vehicles gathered in the area overnight

Tripoli, Libya (CNN) -- Rival Libyan militias battled for Tripoli International Airport on Sunday, with the sounds of gunfire and explosions echoing across the Libyan capital.

Brigades from the Zintan militia have controlled the airport since the Libyan civil war in 2011. Other militias have tried to wrest control of the airport from the Zintan brigades for years.

Armed vehicles gathered in the area overnight before fighting broke out around dawn Sunday, witnesses said.

"The revolutionary brigades are inside Tripoli International Airport," the Libyan Revolutionaries Operations Room, an Islamist militia in the capital, posted on its Facebook page.

"The revolutionary forces have reached the perimeter of Tripoli International Airport ... clashes with some of the armed groups present there," the group said.

One of Libya's two national airlines, Afriqiyah Airways, said on its official Facebook page that it was diverting some of its international arrivals to the city of Misrata and that the airport in Tripoli will be closed for three days.

Late Sunday morning, the sounds of fighting could also be heard in the vicinity of a Zintan base in a western suburb of Tripoli.

Even though Libyans successfully toppled the 42-year rule of Moammar Gadhafi in 2011, the country is far from stable. Armed militias are now the biggest challenge.

The international community has largely agreed that the key to resolving Libya's crisis is national dialogue. But a weak central government -- combined with militias split along regional, tribal and ideological lines -- has added to the challenge.

"The United States is deeply concerned by the ongoing violence in Libya and dangerous posturing that could lead to widespread conflict there," the U.S. State Department said in a statement Saturday.

"We affirm our support for Libya's democratic transition and urge the seating of the new Council of Representatives as soon as possible," the statement added. "Finally, we stress the vital role Libya's Constitution Drafting Assembly plays in building the new country for which Libyans sacrificed so much during the revolution."

CNN's Holly Yan contributed to this report.