Ukraine knew Russia was planning a summer offensive, but not where it would start. That became clear on 10 May, as Russian forces penetrated the border area north of Ukraine’s second largest city of Kharkiv.

They have since grabbed a number of villages not far from the frontier, and are trying to push forward as Ukraine’s outgunned forces try to shore up a weakened front line.

By entering Vovchansk, only 5km (3 miles) inside Ukraine, and seizing large areas of Ukrainian territory in the Kharkiv region, Russian forces may be trying to create a buffer zone to fend off Ukraine’s own cross-border attacks.

Having seen the relatively poor state of Ukraine’s defences, they may also have far more ambitious plans.

Russia’s Vladimir Putin has for some time talked of setting up some kind of “sanitary zone” that would protect the southern Belgorod region from drone or missile attack. Belgorod has also proved vulnerable to cross-border incursions by two Ukraine-based Russian paramilitaries.

Russia could be planning a further cross-border push towards the northern city of Sumy, to the north-west. Ukraine’s military spy chief, Kyrylo Budanov, believes a “small group of forces” is waiting there, ready to act.

Sergei Shoigu, the new head of Russia’s security council, has said only that the military is advancing in all directions.

That could involve pushing deeper into Ukraine, either to force Kyiv to divert forces from the fiercest front line in the eastern Donbas or seize increasing amounts of territory.

The US-based Institute for the Study of War think tank believes the main aim is to create a buffer zone, as does Russian military analyst Anatoly Matviychuk.

But Russian forces are also pushing towards the village of Lyptsi, some 20km from the northern outskirts of Kharkiv and, in his words, “we can practically see the suburbs of Kharkiv through binoculars”. Suddenly capturing Ukraine’s second biggest city could be in Russian sights.