The downing of a Syrian plane by Turkish aircraft on Sunday is a sharp reminder that the regional spillover from Syria's civil war is not just an issue of hundreds of thousands of desperate refugees trekking to neighbouring countries. The danger of armed clashes across mutual borders is rising too.

The incident also highlights the grim lesson that armed conflicts usually intensify when peace negotiations are on the agenda. All sides ramped up their offensives in the run-up to and during the Geneva talks on Syria in January and February, in the hope that military advances would give them stronger bargaining power. That lethal momentum has not slowed.

At the weekend western-supported rebels, along with their jihadist allies, the al-Nusra front, seemed to have captured Kasab – the last border crossing to Turkey still controlled by Bashar al-Assad's forces. The Syrian warplane's bombing runs were part of Assad's counter-attack. In nearby clashes on the ground the president's cousin, Hilal al-Assad – a leader of the National Defence Force militia – was killed on Sunday.

Elsewhere in the north, Syrian forces are slowly managing to capture rebel-held ground around the divided city of Aleppo, and might soon surround it. Meanwhile the US is stepping up its arms supplies to opposition fighters on the southern Jordanian front.

The government's indiscriminate artillery and air attacks on rebel parts of Aleppo, and the villages that lie north of it, have produced a huge surge of refugees. In the Turkish border town of Kilis, aid officials told me that a camp they help just inside Syria hosted 10,000 refugees at the beginning of this year. Now it hosts 50,000.

Turkey has shouldered an amazing burden, virtually without complaint. Kilis has a Turkish population of 80,000. Almost 40,000 Syrians now live there as well. Some are there because they are not given space in any of the three local camps. Others prefer a flat, at least until their funds for rent run out. Many are then forced to risk going back to Syria. At the border I approached four families at random. Two were returning to Aleppo. Abu Mohammed cradled a three-month-old baby in his arms. His burqa-clad wife stood a few yards off as he said: "We left Aleppo four days ago because of fierce bombing night after night. We're thinking of going back because at least we have our own home and it's not yet destroyed."

Abu Omar was standing beside massive piles of suitcases, bulging black plastic bags and rolled up mattresses. His wife was sitting on one of the piles, their four children beside her. They had been in Kilis for four months, he said. Their home village of al-Bab recently fell under the control of fighters from the Islamic State of Iraq and the Levant (which is in constant conflict with other rebel groups), and they had heard that clashes were over and it was safe to return.

Syria's war is a strategic stalemate that no side can win, but the ebb and flow of the battlelines creates waves of misery that confront civilian families every day with the need to make life-and-death decisions about staying, moving or returning. The wrong choice can be fatal.

Hoping for a resumption of the Geneva talks seems futile, as the chief UN negotiator, Lakhdar Brahimi, acknowledged on Monday. The west's shrill and short-sighted reaction to Russia's takeover of Crimea has poisoned relations with the Kremlin and undermined any chance of US-Russian co-operation over Syria for several months at least. It will also have reinforced the Kremlin's determination not to see Assad toppled. Similarly, the west's refusal to invite Iran to join the Geneva process remains in force, and this only increases Tehran's support for Assad.

So the picture is bleaker by the day. The view among analysts in Turkey, as well as among Syrian refugees I talked to, is that this horrendous war could last another five or even 10 years.

Jonathan Steele is a former chief foreign correspondent for the Guardian