Clashes between forces loyal to the Yemeni regime and opposition fighters from a key tribe continued in Sanaa on Thursday, as the country reportedly moved closer to outright civil war.

Amid reports of heavy shelling around the city and government warplanes circling above, flights in and out of the capital were suspended. Thousands of armed men were reportedly moving toward Sanaa, in the latest escalation of violence between forces loyal to President Ali Abdullah Saleh and fighters from the powerful Hashid tribe, ABC Online reported.

Tribal leaders claim hundreds more fighters are mobilizing to support chief Sheikh Sadeq al-Ahmar, the leader of the influential Hashid tribal federation, the BBC reports.

Medics in Sanaa reportedly said that 15 people, including a seven-year-old girl hit by a stray bullet, had been killed in overnight clashes in the Yemeni capital.

According to witness reports late Thursday, government troops, including elite Republican Guard units, had confronted the tribal fighters at a key checkpoint north of Sanaa. The capital was split, with Yemeni security forces holding the southern part of the city against tribesmen and renegade military units in the north, VOA reports.

Fighting also continued In the southern city of Taiz, where over 50 people have died since the weekend, and the coastal town of Zinjibar, where Saleh's forces are pounding a suspected Al Qaeda-led insurrection.

Meanwhile, Saleh has deployed U.S.-trained Yemeni counterterrorism forces to engage opposition troops commanded by General Ali Mohsen al-Ahmar — the country's most prominent military defector, the Wall Street Journal reports. U.S. officials have warned Saleh — who receives aid from Washington for its fight against Al Qaeda in the country — not to use U.S.-trained forces or weapons against protestors.

The Obama administration dispatched a top counterterrorism official, John Brennan, to Saudi Arabia and the UAE in order to increase pressure on Saleh to sign a Gulf-brokered transition agreement — which he has balked at three times already — Reuters reports. The United States and its allies in the region fear the chaos in Yemen could spill over to neighboring countries and threaten strategic oil supply lines.

The latest outbreak of fighting in the capital broke began Tuesday after a truce collapsed between security forces and tribesmen who have taken control of public buildings across the capital, ABC reports. The truce was announced May 27, after Saleh — warning of a civil war — refused to sign a Gulf-brokered plan for him to give up office.