Syrian opposition factions failed on Monday to meet a deadline to leave the planned buffer zone in the northwestern province of Idlib. The development cast fresh doubt over a Turkish-Russian deal to avert a regime offensive against the region. The truce agreement reached nearly a month ago for Idlib gave "radical fighters" until October 15 to leave a proposed demilitarized area between regime and opposition forces. The accord was a last-ditch effort to stave off a regime onslaught on Idlib, the largest opposition stronghold left in war-ravaged Syria and home to around three million people. But the target date for the withdrawal came and went without any hardliners leaving. "We did not document the withdrawal of any extremist fighters from the entire demilitarized zone," Rami Abdel Rahman, the head of the Syrian Observatory for Human Rights war monitor, said on Monday morning. Extremists had until midnight Sunday to Monday to pull out, according to Abdel Rahman and two faction commanders in Idlib. Hours before the cut-off time, Idlibs Hayat Tahrir al-Sham extremist group vowed to continue fighting. "We have not abandoned our choice of fighting towards implementing our blessed revolution," said HTS, an alliance led by Al-Qaedas onetime Syria branch. Their withdrawal was seen as the real test of the accord reached on September 17 between opposition backer Ankara and regime supporter Moscow in the Russian resort town of Sochi. The deal provides for a 15-20 kilometer buffer zone semi-circling opposition-held areas in Idlib and the neighboring provinces of Latakia, Hama, and Aleppo. It set a first deadline of October 10 for all opposition factions and extremist groups to pull heavy weapons from the zone, a task which Turkey, the Observatory, and the opposition said was done on time. But late Saturday, mortar rounds fired from the buffer hit regime positions and killed two soldiers, the Observatory said, indicating heavy arms may still be in the zone. "Even if the agreement is not fully implemented today, it doesnt mean that its not holding," said Haid Haid, research fellow at the International Center for the Study of Radicalization at Kings College in London. HTS would seek the "best possible scenario" for its own survival, but that may include "a partial withdrawal." Sam Heller, an International Crisis Group analyst, said the ambiguity in HTSs statement "could be seen as a sort of internal acceptance of the Sochi deal and its implementation". The onus was now on Ankara and Moscow -- the deals two sponsors and the chief powerbrokers of the Syrian war -- to overcome the missed deadline. Foreign Minister Sergei Lavrov said the buffer should be fully operational by October 15, but that a delay of one or two days would "make no difference." "In any case the quality of the work is more important... We actively support the efforts of our Turkish partners," he said.