BEIRUT — A U.N. official said Tuesday that recent fighting in northern Syria has displaced tens of thousands of people – some of them staying in open fields with nowhere to go – as clashes between Russian-backed Syrian government forces and opposition fighters intensified.

Activists meanwhile said that Russian airstrikes killed at least 45 people in a Syrian coastal province, including a rebel commander who formerly served in President Bashar Assad’s army. If confirmed, it would make it one of the deadliest incidents since Russia began its aerial campaign in Syria Sept. 30.

The Russian airstrikes have allowed Syrian troops and their allies from Lebanon’s Hezbollah militant group to launch multiple ground offensives in northern, central and southern Syria as well as in the rebel-held suburbs of the capital, Damascus.

That has increased pressure on Western-backed rebels and pushed more civilians out of the areas of fighting, potentially creating a fresh wave of refugees.

A U.N. official in Geneva said that around 35,000 people are reported to have been displaced over the past few days.

“Many people are living with host families and in the informal settlements in areas west of the city,” said Vanessa Huguenin, a spokeswoman for the Office for the Coordination of Humanitarian Affairs. “People urgently need food and basic household and shelter items,” she said in an e-mailed statement, adding that aid agencies are increasingly concerned for families living outdoors as the weather gets colder.