The United Nations’ envoy for Syria, Staffan de Mistura, speaks to the press as he heads for Damascus on Tuesday. (Louai Beshara/AFP/Getty Images)

Russia on Tuesday denied reports that its warplanes carried out a deadly strike on a hospital in northern Syria the previous day, complicating efforts to broker a cease-fire as the United Nations’ top Syria envoy arrived in Damascus to push for greater humanitarian access.

Kremlin spokesman Dmitry Peskov called the reports that Russian planes had struck the hospital “unfounded accusations” and pointed to a statement from Syria’s ambassador to Russia that U.S. warplanes were responsible. The United States is conducting airstrikes in Syria against suspected Islamic State targets.

U.S. planes were not flying in the region, nor were those of any U.S. partners, Pentagon spokesman Peter Cook said Tuesday. State Department spokesman Mark Toner blamed Russia for the airstrikes.

At least seven people were killed in the strike Monday morning, according to the nonprofit aid group Doctors Without Borders, which runs the hospital. U.N. human rights spokesman Rupert Colville said the strike “may amount to a war crime.”

[Nearly 50 people killed in strikes on hospitals and schools in Syria]

A photo released by Doctors Without Borders shows Syrians gathering outside a hospital after the building was hit by air strikes. (Sam Taylor/AFP/Getty Images)

The statements came as the U.N. special envoy for Syria, Staffan de Mistura, met with Syria’s foreign minister in the capital, Damascus, on Tuesday. De Mistura pressed for “unhindered” access to besieged populations across Syria, spokeswoman Jessy Chahine said.

The U.N. envoy said later in a statement that he had a “useful meeting” with Syrian Foreign Minister Walid al-Moualem.

The priority for the United Nations is “humanitarian access to besieged areas, by anyone who is besieging it,” de Mistura said, adding that it was “the duty of the government of Syria” to reach Syrian civilians under siege.

In northern Syria, at least two schools and four hospitals were hit by airstrikes on Monday, according to the United Nations.

UNICEF said Tuesday it was still trying to assess the impact of strikes on two hospitals in the north the day before. Video footage of the aftermath at one of the facilities — a children’s and maternity hospital — showed babies crying in incubators, their monitor alarms ringing.

“UNICEF staff are working with our partners to verify the number of casualties, including children killed and injured in the attacks,” spokesman Kieran Dwyer said.

Colville, in Geneva on Tuesday, called the incident “completely outrageous,” the Reuters news agency reported. “All the norms and rules and standards on conduct of warfare have just been swept aside in Syria,” he said.

The fighting has intensified in the country since Syrian government forces — including Iraqi and Iranian fighters backed by Russia — advanced on rebel positions in the north this month.

Russia intervened in the five-year-old conflict to prevent its ally, Syrian President Bashar al-Assad, from being toppled by rebels.

The government’s gains came as Russia and the United States met in Geneva for peace talks that swiftly collapsed. De Mistura is working to bring stakeholders back to the table for a new round of talks starting Feb. 25.

“We are witnessing a degradation on the ground that cannot wait,” U.N. spokesman Ahmad Fawzi said Tuesday in Geneva, according to Reuters.

[Battered Aleppo latest stage for Syria’s proxy war]

But in comments carried by the state-run Syrian Arab News Agency, Assad said it is unlikely the fighting will stop.

“Regarding a cease-fire, a halt to operations, if it happened, it doesn’t mean that each party will stop using weapons,” he said.

Hashim al-Moussawi, a spokesman for the Iraqi Shiite militia Harakat al-Nujaba, which has fighters battling for Assad in Syria, said the pro-government forces were in a position of strength. “We’ve proved that we still have the upper hand,” he said.

Moussawi said it was “too early” to talk about a cease-fire.

“There’s a war,” he said.

Morris reported from Baghdad. Erin Cunningham in Cairo and Daniela Deane in London contributed to this report.

Read more:

The current chaos in Syria may help the Islamic State

Syrian rebels are losing Aleppo and perhaps also the war

Today's coverage from Post correspondents around the world