ALEPPO, Syria — A Syrian warplane crashed in flames in eastern Syria in disputed circumstances on Monday as President Bashar al-Assad's forces pursued their drive to quell rebels in Aleppo and elsewhere.

State media said technical problems had caused the crash and a search was under way for the pilot who had ejected from the aircraft during a "regular training mission."

Hours earlier, rebels said they had shot down a plane in the same area and an opposition activist who said he was in the northeastern town of Mohassen posted a video of a fighter jet bursting into flames while under heavy ground fire.

The video could not be independently verified. The downing of a warplane would be a rare coup for lightly armed rebels up against the superior weaponry of Assad's forces, which have made increasing use of air power in recent months.

Reuters journalists have seen fighter jets firing rockets at rebel-held villages and districts in the northern city of Aleppo, the main battle front in Syria so far this month.

Fawaz Zakri, a member of the opposition Syrian National Council visiting rebel fighters in Aleppo, urged outside powers to send arms and ammunition to help Syrians defend themselves.

"We are not asking for tanks or artillery launchers. We need anti-aircraft weapons and rockets so that we can stop this takeover by the Assad army and daily shelling," he said, toting a black rifle near front lines in the Saif al-Dawla district.

Saudi Arabia and Qatar are believed to be paying for weapons reaching rebels via Turkey, while the United States and Britain say they are stepping up non-lethal assistance. Assad's main military suppliers are thought to be Russia and Iran.

BODIES IN STREET

Four civilians lay dead in a nearby street in Saif al-Dawla, next to the bitterly contested Salaheddine district. All were victims of army sniper fire, the rebel Free Syrian Army said.

The bodies of a man and a woman slumped in a bullet-riddled yellow taxi. A bloated male corpse lay on a stinking pile of rubbish. Another dead man sprawled in the street, his face purple and bloated, with several dolls lying near his hand.

Syria's uprising began in March last year with peaceful demonstrations for change, inspired by Arab revolts elsewhere. Assad's violent response spawned an insurgency, plunging Syria into a civil war in which more than 18,000 civilians, soldiers and rebels have been killed, opposition sources estimate.

More than 50 Syrians, including 39 civilians, were killed across the country on Monday, according to an opposition watchdog, the Syrian Observatory for Human Rights. Daily death tolls have ranged far higher than this in recent weeks.

The head of a faltering U.N. monitoring mission said violence was intensifying across Syria and accused both sides of ignoring the plight of civilians caught up in the conflict.

"The indiscriminate use of heavy weapons by the government and targeted attacks by the opposition in urban centers are inflicting a heavy toll on innocent civilians," General Babacar Gaye said in Damascus. "I deeply regret that none of the parties has prioritized the needs of civilians.

The mandate for the U.N. monitors, whose original mission was to observe an April ceasefire that never took hold, expires on August 19. Their numbers have already been cut to a third because violence has made it impossible for them to move around.

The relentless conflict and disagreements among big powers and regional rivals foiled diplomatic efforts led by U.N.-Arab League envoy Kofi Annan, who resigned in frustration.

The United States and Turkey indicated at the weekend they would consider ways of helping the Syrian opposition, with a no-fly zone among many options, and preparing for a post-Assad era.

Russia and China firmly oppose outside military intervention in Syria, not that Western powers have until now shown any stomach for any Libya-style air campaign or other overt action.

LOOSE TALK

Germany said it was unwise even to raise the subject.

"We believe that the discussion about a military intervention could exacerbate things because it could increase the danger of the situation in the entire region getting out of control," a Foreign Ministry spokesman said in Berlin.

"We continue to work on bringing about a change of heart in Russia and China that would enable decisive action by the U.N. Security Council," he said, alluding to past vetoes by Moscow and Beijing of Western-backed resolutions on Syria.

China said it would host Assad aide Bouthaina Shaaban on Tuesday and consider separately inviting opposition figures as part of efforts towards resolving the crisis.

Syria's ambassador to Iran said Damascus would welcome dialogue with opposition groups, as long as Assad supervised the talks — making them a non-starter for rebels.

Hamed Hassan said his country, under attack from "terrorist groups," had co-operated with Annan's peace plan, but that other countries had scuppered it, Iran's state news agency reported.

With diplomacy deadlocked, even efforts to bring aid to suffering Syrian civilians in war zones have stuttered.

U.N. humanitarian chief Valerie Amos will go to Syria on Tuesday to discuss how to increase emergency aid, but diplomats said fighting must ease to allow access to trouble spots.

"Two million people are now estimated to have been affected by the crisis and over one million have been internally displaced," a U.N. statement said.

More than 4,000 Syrians have fled to Turkey in the past three days, bringing the total there to almost 60,000, Turkish authorities said. Nearly 150,000 registered refugees from Syria are now in Turkey, Lebanon, Iraq and Jordan, U.N. figures show.