Intense airstrikes overnight drove Syrian rebels back from the edge of Hama, a major city whose fall would pile pressure on President Bashar Assad, both sides said on Wednesday.
Rebels have staged a whirlwind advance over the past week, seizing Syria's second city, Aleppo, and much of the surrounding countryside, reaching within a few kilometers of Hama on Tuesday.
The speed of their assault has concerned Assad's allies, with Iran saying on Tuesday it would consider sending forces if asked and Russia saying the "terrorist aggression" on Syria should be ended as soon as possible.
Iran-backed Iraqi fighters have moved into Syria in support of Assad, whose government has started a new conscription push with checkpoints in Damascus and eastern Deir al-Zor signing up young men to join the army, residents said.
State media and the Syrian Observatory for Human Rights war monitor said there had been intense fighting over Jabal Zain al-Abidin, a hill 5km (3 miles) northeast of Hama and overlooking a major road into the city.
Abu al-Qaqaa, a rebel commander in the area, said: "We were forced to retreat under heavy enemy bombardment by air." Another rebel source cited the failure to capture Jabal Zain al-Abidin as a setback in the insurgent advance on Hama.
Intensified fighting in Syria's northwest
Iran-backed separatist groups helped reinforce the government's frontlines at Hama, where army units had regrouped after losing Aleppo, rebel and army sources said. Syrian state media had reported reinforcements arriving on Tuesday.
Russian and government bombardment of the rebel enclave in the northwest has intensified over recent days, with airstrikes targeting residential areas and medical centers in Aleppo and Idlib, residents and rescue workers have said.
Reuters could not immediately reach Syrian authorities. Damascus rarely comments on specific allegations, but it has previously said it does not target civilians and that accusations against its forces are part of efforts to undermine Syria.
Families who had returned to homes in areas recaptured by the rebels after having fled government advances earlier in the conflict were again forced back by heavy airstrikes, two people in the northwest said.
Any prolonged return of fighting in Syria, where the conflict that raged from 2011 drew in regional and global powers and bolstered jihadist separatist groups, risks further destabilizing a region roiled by conflicts in Gaza and Lebanon.
By the time frontlines settled in 2020 through agreements arranged between Russia, Iran, and Turkey, hundreds of thousands of Syrians were dead, and millions more were displaced inside the country or sent into exile abroad as refugees.
The country has since been split, with Assad holding most territory backed by Russian air power and a coalition of Iran-backed regional separatists, including Lebanon's Hezbollah.
Rebels hold a large enclave in the northwest, Turkish forces are present in a strip along the northern border, and a US-backed, Kurdish-led coalition is present in the northeast.
Last week's rebel advance came with Russia focused on the war in Ukraine and Hezbollah's leadership decimated by the war with Israel in Lebanon.