BEIRUT — The United Nations appealed for an immediate cease-fire in Syria on Tuesday as spiraling violence pushed the country to the brink of one of the worst humanitarian crises in the seven-year war.

A halt to the fighting for at least a month is vital to allow urgently needed aid to reach 2.9 million stricken people living around the front lines of the latest fighting, the U.N. mission in Damascus said, warning of “dire consequences” if the current levels of violence are sustained.

The appeal coincides with the collapse in recent weeks of a year-old Russian effort to tamp down the violence through “de-escalation zones,” which had helped contribute to a perception that the war in Syria finally was winding down.

Instead, the first weeks of 2018 have turned into one of the bloodiest periods of the conflict yet, with hundreds killed in ­airstrikes, nearly 300,000 displaced in northwestern Syria and 400,000 at risk of starvation in a besieged area east of Damascus that has not received food since November.

“The war is far from over,” Panos Moumtzis, the United Nations regional humanitarian coordinator for the Syria crisis, said at a briefing for journalists in Beirut. “This is a really critical stage. The humanitarian situation has dramatically deteriorated, and that’s why we are ringing alarm bells.”

There has also been a spike in the number of reports of attacks by the government using chlorine as a chemical weapon, prompting warnings from the United States to the Syrian government to desist and to Russia to pressure its ally to halt the attacks. There have been six reported attacks using bombs laden with chlorine in the past month, the State Department said, adding that Washington is “gravely alarmed” by the continued allegations of the use of chlorine gas.

The latest surge in violence began in late December and coincides with the winding down of the fight against the Islamic State in eastern Syrian, which freed up thousands of Syrian government forces to take on rebels and al-Qaeda-allied fighters in their last few enclaves elsewhere in the country.

Since then, nearly 300,000 civilians have fled a new government offensive to recapture territory in the northwestern province of Idlib. They have taken refuge in the north of the province in one of the biggest displacements of the war so far, Moumtzis said. Aid workers are struggling to find space to accommodate them, and as many as 750,000 more could flee if the violence continues, he said.

A separate crisis is developing east of Damascus, in the rebel-held enclave of Ghouta, where over 400,000 people surrounded by government forces have been reliant on U.N. aid for the past four years. The Syrian government has prevented all deliveries of food to the area since November, putting the population at risk of starvation, and has refused to allow the evacuation of about 600 people injured in the fighting to hospitals in nearby Damascus, Moumtzis said.

Syrian warplanes pounded the area Tuesday, conducting over 40 strikes and killing at least 37 people, according to the Britain-based Syrian Observatory for Human Rights, which monitors violence in Syria. Activists in the area later said more than 70 people had died in the raids.

Later in the day, five civilians died when rebels fired shells into government-controlled neighborhoods in Damascus, including three in the historic Bab Touma district of the Old City, according to the official Syrian news agency SANA.

Those strikes followed an onslaught of airstrikes by Russian and Syrian warplanes against rebel-held towns and villages in Idlib on Sunday and Monday, in apparent retaliation for the downing of a Russian warplane in the province Saturday. At least three hospitals or health facilities were hit in the strikes, which recall some of the worst periods of the war.

The Syrian Network for Human Rights said that at least 260 civilians had died in airstrikes in Syria since December, including 88 children and 71 women. The Observatory said that more than 500 people had died in the recent upsurge of violence.

“We’re in emergency mode,” Moumtzis said as he described the numbers of people who are hungry, displaced or in need of immediate medical treatment. “There’s a dramatic deterioration . . . and the situation is desperate for the people on the ground.”

The incursion by the Turkish army into the Kurdish-
controlled enclave of Afrin has further complicated the war, displacing more than 15,000 people and adding new obstacles to efforts to resolve the war.