DAMASCUS, Syria — Islamic State militants recaptured a vital border crossing in northern Syria and shot down a government warplane in the country's west Monday as the U.N.'s special envoy urged the warring parties to respect a fragile cease-fire ahead of peace talks set to resume in Geneva this week.
U.N. Special Envoy Staffan de Mistura's plea came amid stepped up fighting around the northern city of Aleppo, Syria's largest, and elsewhere in the country's northern and western provinces.
He spoke after meeting with Syrian Foreign Minister Walid al-Moallem in Damascus in preparation for the talks, set to begin Wednesday in Geneva between the government and an umbrella opposition coalition backed by the U.S., Saudi Arabia, and other Western powers.
De Mistura said he emphasized the importance "of protecting and maintaining and supporting the cessation of hostilities," describing it as fragile and stressing that all sides "need to make sure that it continues to be sustained."
The U.N. envoy said the talks would focus on a political transition for Syria, where the civil war, now in its sixth year, has killed 250,000 people and displaced half the country's pre-war population of 23 million people. About 4 million people have fled the country.
"We will be focusing in particular on political transition, on governance and constitutional principles," de Mistura said. "We hope and plan to make them constructive ... and concrete."
Al-Moallem called for a dialogue "without preconditions," a reference to opposition demands that President Bashar Assad step down and be excluded from any transitional government. He said the government delegation would arrive Friday, two days after parliamentary elections are to be held in Syria. The vote — expected to be a rubber stamp of Assad loyalists — will only take place in government-controlled areas as the Damascus authorities are unable to organize any balloting in territory under rebel or Islamic State control.
Meanwhile, opposition activists reported clashes and government air raids near Aleppo, where rebel factions alongside al-Qaida's Syria affiliate, the Nusra Front, seized territory from the government earlier this month.
Russia's military announced Monday it would help the Syrian army battle back the Nusra Front, but "there is no plan to storm Aleppo."
Lt. Gen. Sergei Rudskoi of the military's general staff said Nusra has nearly 10,000 fighters southwest and north of Aleppo and plans to cut a strategic highway linking the city with the rest of the country. Rudskoi said the Syrian army backed by Russian warplanes is taking action to derail Nusra's plan.
In Washington, U.S. State Department spokesman Mark Toner said Secretary of State John Kerry, speaking to his Russian counterpart Sergei Lavrov in a telephone call Sunday, expressed concern over the recent increase in violence in Aleppo and elsewhere aimed not just at IS and the Nusra Front, which are excluded from the U.S.- and Russian-brokered cease-fire, but at rebel groups that are covered by it.
"We are concerned about plans to attack and seize control of Aleppo, when there are clearly opposition groups there that are part of the cessation of hostilities," Toner said.
Russia's air force was instrumental to reviving Assad's military command in the months leading to the cease-fire that came into effect in late February and has brought relative peace to the country for the first time in the civil war.
Meanwhile, IS militants shot down a Syrian war plane during violent clashes Monday west of Damascus, the Britain-based Syrian Observatory for Human Rights said. It said the plane was shot down in the vicinity of the Dumayr Air Base in the eastern Qalamoun mountains, which straddle the border with Lebanon.
A news agency for the extremist group said the wreckage landed inside the base, damaging three other planes. The Aamaq News Agency posted a video on social media showing thick smoke emerging from what it said was the Dumayr base.
Earlier Monday, IS militants captured the strategic town of al-Rai on Syria's border with Turkey after intense fighting, the Observatory said. The town serves as the Islamic State group's access point to supply lines and also sits along the road to the IS stronghold in Aleppo province.
The latest IS gains show the group is still capable of launching counterattacks as it comes under pressure on different fronts in Iraq and Syria. IS has lost wide areas in Iraq and Syria recently, including the historic central town of Palmyra that was captured by Syrian government forces and their allies.
The extremists have also suffered leadership setbacks as U.S. drone strikes in Syria have killed several top IS and Nusra Front commanders and key figures in recent weeks.
Mroue reported from Beirut. Associated Press writers Jamey Keaten and Philip Issa contributed to this report from Geneva and Beirut.