Syrian forces are pressing a military offensive and laying mines near the border with Turkey in an attempt to block a flow of refugees and supplies for insurgents, rebel activists and a Turkish official at the frontier said.
Syrian army activity, visible across olive groves from the small Turkish border village of Bukulmez, comes days before a ceasefire deadline agreed by President Bashar al-Assad.
The flow of refugees to Turkish camps nearby swelled to 2,800 on Thursday as violence in the bordering Idlib province worsened.
It is impossible to verify reports from the many refugees fleeing Syria since foreign correspondents' access to the country is strictly limited by the Damascus government.
A Syrian helicopter could be seen hovering over mountains on the Syrian side of the border in clear view of refugees at a camp. A Reuters television journalist with experience in the area said it was the first time since the crisis began that he was aware of Syrian aircraft flying close to Turkey.
Villagers reported hearing artillery along the border.
A Turkish foreign ministry official touring the camps in the area said there was new activity close to the border.
"The Syrians have been mining the border, especially the southern Idlib part which has been restricting the flow of refugees," the official said. He declined to give his name.
Activists said mining was concentrated on southerly parts of Turkey's border with Syria, from the town of Harem westwards to the coast.
"Assad is using the days granted to him by the international community to choke off the refugee movement to Turkey and the delivery of any kind of aid," said Muhammad Abdallah, a rights campaigner from Idlib.
Still, refugees were getting through, the flow rising to 2,800 on Thursday.
Mr Assad says his government is under attack from foreign-backed Islamist militants and denies his own troops have targeted civilians. He says support from Western and Arab governments for the rebels is only feeding the violence and obstructing a peaceful settlement.
In Ankara, Turkish Foreign Minister Ahmet Davutoglu demanded Mr Assad keep his promise to cease military operations.
"At the moment the number of refugees to have entered Turkey is 23,835. If more refugees come then the UN and international community must take action," he told reporters.
Under an internationally backed plan agreed with Damascus, government forces should cease operations and withdraw from settlements by 10 April. Rebels should then cease fire within 48 hours.
UN special envoy Kofi Annan said on Thursday he had been told by Damascus that troop withdrawals were under way from Idlib, as well as Zabadani and Deraa. But UN Secretary General Ban Ki-moon said on Friday the conflict was worsening and attacks on civilian areas persisted.
Turkey fears that a complete breakdown in Syria would unleash a flood of refugees reminiscent of the half million who descended on Turkish territory from Iraq during the Gulf War in the early 1990s.