By Samia Nakhoul and Mohammed Ghobari

SANAA (Reuters) – Heavy explosions rocked the Yemeni capital Sanaa in the early hours of Thursday as fighting to topple the veteran president, Ali Abdullah Saleh, threatened to descend into civil war.
More than 40 people have been killed since Monday in a part of northern Sanaa where fighters loyal to powerful tribal leader Sadiq al-Ahmar have been attacking and trying to take over government buildings including the Interior Ministry.
A Reuters correspondent was woken after midnight by the latest blasts.

"The explosions can be heard across town in the south of Sanaa. This seems to be heavier weapons than the machineguns and the mortars of the past few days," one resident said.
Each side blamed the other for the violence, which the opposition said could start a civil war.
The United States and Saudi Arabia, both targets of foiled attacks by a wing of al Qaeda based in Yemen, have tried to defuse the crisis and avert any spread of anarchy that could give the global militant network more room to operate.

Washington ordered all non-essential diplomats and embassy family members to leave the country.
"The security threat level in Yemen is extremely high due to terrorist activities and civil unrest. There is ongoing civil unrest throughout the country and large-scale protests in major cities," the State Department said.
Citizens were fleeing the capital to escape the fighting in the Hasaba area of Sanaa, which erupted a day after Saleh pulled out for the third time from a Gulf Arab-brokered deal for him to step down and make way for a national unity government.

Pressure has been mounting since February, when protesters inspired by revolutions in Tunisia and Egypt began camping in squares and marching in their hundreds of thousands to call for Saleh to go.
Saleh's attempts to stop the protests by force have resulted in hundreds of deaths.

CITY AT WAR

Saleh, a wily political survivor, said Wednesday he would make no more concessions to those seeking his departure. But the capital of the country of 23 million has begun to feel like a city at war.
Fighters in civilian clothes roamed some districts on Wednesday and machinegun fire rang out sporadically.
Electricity was intermittent and Sanaa's airport was closed. Many city-center streets were deserted in the afternoon, but for government checkpoints.

Long lines of cars snaked out of the city, bags piled high on their roofs, even as gunmen blocked entrances to prevent tribesmen from bringing in reinforcements, witnesses said.
"It's no longer possible to stay in Sanaa. The confrontations will reach all parts of the city," said Murad Abdullah as he left by car. "I am afraid for my life. I will go to my village in Ibb. The situation there is safe."

Witnesses and officials said supporters of Ahmar, head of the Hashed tribal federation to which Saleh's Sanhan tribe also belongs, controlled several ministry buildings near Ahmar's compound including the trade and tourism ministries, as well as the offices of the state news agency Saba.
Ahmar's fighters also attacked the main building of the Interior Ministry, whose courtyard came under fire from rocket-propelled grenades, witnesses said.

Televised images of Ahmar's own compound showed tribesmen rushing through opulent but dusty halls, their floors spattered with blood, as they helped colleagues wounded in the fighting.
Saleh told a group of invited reporters including a Reuters correspondent Wednesday that his government was "steadfast."

"We are bearing the shocks of what happened from the sons of al-Ahmar: the chaos and the attacks on state institutions, the press and the Ministry of Industry and the Ministry of Interior. This is a provocative act to drag us into a civil war...

"We are contacting some people to talk to them and persuade them to stop trying to storm the Interior Ministry and opening fire at the ministry in order to avoid widening the conflict."
Saba said four civilians had been killed and 11 injured in Wednesday's fighting.

"THIRSTY FOR BLOODSHED"

General Ali al-Mohsen, a regional army commander who has sided with protesters, called on the armed forces to defy the president.
"Beware of following this madman who is thirsty for more bloodshed," he said.
Shadi Hamid, analyst at the Brookings Doha Center, said: "I think there's a real risk that violence can escalate, and we see a move toward low-intensity civil war.
"There's a real loss of faith in the political process after Saleh refused to sign a deal several times. That really cast doubt on whether Saleh has any real commitment to letting go of power voluntarily."
Saleh said the deal remained on the table, despite his repeated failure to sign:
"I am ready to sign within a national dialogue and a clear mechanism. If the mechanism is sound, we will sign the transition of power deal and we will give up power ...

"No more concessions after today," he said.
U.N. Secretary-General Ban Ki-moon's spokesman Martin Nesirky said Ban was deeply troubled by the clashes in Sanaa and called for further peace efforts and an immediate end to the fighting, while Britain reiterated calls on Saleh to sign the exit deal.
(Additional reporting by Mohamed Sudam and Khaled al-Mahdi in Sanaa and Nour Merza in Dubai; writing by Cynthia Johnston, Firouz Sedarat and Kevin Liffey; editing by Mark Trevelyan and Sanjeev Miglani)