Syria's army and its Russian and Iranian allies are closing in on major rebel strongholds in the country's north in an advance that has already derailed peace talks and may also unleash a new wave of refugees into Europe.
The military forces of President Bashar Assad, supported by Russian air power and Iran-backed Hezbollah militants, are only about three kilometers (1.9 miles) away from Aleppo, said Rami Abdurrahman, head of the U.K.-based Syrian Observatory for Human Rights, a breakthrough that has cut off rebel forces there from vital supply lines to Turkey.
The United Nations on Wednesday suspended its long-awaited peace conference in Geneva just days after it began, as opposition groups called for international pressure to halt the government advance. NATO's secretary-general, Jens Stoltenberg, said on Friday that the Russian strikes, which mainly targeted opposition groups, were "undermining the efforts to find a political solution to the conflict."
The intensification of fighting around Aleppo, once the country's most populous city, provided an ominous backdrop for world leaders gathered in London to discuss aid to Syria as it could add to mass exodus of refugees that is already destabilizing several European countries.
Turkish Prime Minister Ahmet Davutoglu, speaking in London on Thursday, said as many as 70,000 Syrians were already on their way to Turkey from the northern Aleppo region, and warned of a wider humanitarian disaster if Aleppo city falls. The SOHR said about 40,000 people have fled the region in the past few days.
"In the last three days, Aleppo came under attack from Russian warplanes as well as regime force and foreign fighters fighting along them," said Davutoglu. "They are trying to siege Aleppo and condemn it starvation."
The number fleeing could surpass 100,000 if the rebels can't restore links between Aleppo and Azaz, near the Turkish border, cut off by heavy bombardment by Russian warplanes, said Mehmet Emin Arslan, an official at the Humanitarian Aid Foundation in Turkey's Gaziantep province, near the Syria border.
Syrian state-run television reported on Friday that government forces and allied militias have also overrun territory in the southern province of Daraa, a strategically important city between Damascus and the border with Jordan.
The European leaders in London were seeking an increase in aid that will improve conditions in camps and convince refugees not to risk sea and land journeys to reach European Union countries. British Prime Minister David Cameron said $5.6 billion was pledged for 2016 with a further $5.1 billion committed for the following four years.
"The German government is convinced that the great migration of refugees can only be resolved by addressing its root cause in the region," German Chancellor Angela Merkel, under intense pressure at home to act after more than a million asylum seekers made their way to Germany last year, said at the conference. The meeting is a "significant building block in coming closer to this goal, when one thinks of Syria and its neighbors," she said.
A civil war that has killed 250,000 people over five years and forced millions of others to flee their homes has confronted Europe with the rising threat of terrorism as well as a growing migrant crisis. Turkey, a candidate to join the European Union, is home to the largest Syrian refugee population, and the EU has offered a 3 billion euro ($3.4 billion) aid package to encourage Turkey to keep refugees from heading west.
Merkel will meet with Davutoglu in Ankara on Monday for talks, government spokesman Steffen Seibert said in Berlin, adding that Assad's advance on Aleppo is a matter of "great concern."
Taking Aleppo, Syria's former commercial hub, would give Russia, Iran and Assad more bargaining power at any future settlement talks and more say in how the region will be redefined. It would also exacerbate tensions with Turkey, which supports the ouster of Assad and last year shot down a Russian fighter plane it said crossed into its territory during operations over Syria.
A siege of the city and capture of the northern supply line "would effectively diminish Turkey's say over the war in Syria, while leaving it exposed to a growing refugee crisis," Nihat Ali Ozcan, an analyst at the Economic Policy Research Foundation of Turkey, an Ankara-based think-tank, said by phone.
Russia warned on Thursday that Turkey may be preparing to intervene militarily to stop that from happening. The Russian Defense Ministry said it has "reasonable grounds" to suspect Turkey of "intensive preparations" for sending troops into Syria, though there was no immediate evidence of that on the Turkish side.
Dana Khraiche, James G. Neuger, Thomas Penny and Arne Delfs contributed.