DAMASCUS — 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 UN’s special envoy urged the warring parties to respect a fragile cease-fire ahead of peace talks set to resume in Geneva this week.

UN Special Envoy Staffan de Mistura’s plea came amid stepped-up fighting around the northern city of Aleppo, Syria’s largest, and elsewhere in 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 United States and other Western powers and by Saudi Arabia.

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 UN 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 prewar 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.’’

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.

Islamic State fighters captured the town of al-Rai on Syria’s border with Turkey after intense fighting, the Britain-based Syrian Observatory for Human Rights said. The town serves as the group’s access point to supply lines and sits along the road to the militant stronghold in Aleppo province

Islamic State, also known by the acronyms ISIS and ISIL, shot down a Syrian war plane Monday west of Damascus, the Observatory said.

Opposition activists in Syria also reported clashes and government air raids near Aleppo.

Russia’s military announced Monday it would help the Syrian army battle back the Nusra Front, but ‘‘there is no plan to storm Aleppo.’’

Lieutenant General 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, a State Department spokesman said Secretary of State John Kerry, speaking to his Russian counterpart, Sergey Lavrov, in a telephone call Sunday, expressed concern over the recent increase in violence.