Image copyright TASS Image caption This video shows the Russian-backed convoy passing by heavily armed Ukrainian soldiers
Myles Alexander Downing explains how tension is rising along Ukraine’s border with Russia
On Friday, a Ukrainian army unit raided two border posts in Ukraine and seized 11 armoured personnel carriers and 45 trucks of grain.
The vehicles had come from Russia, and according to Ukrainian officials were captured after a brief battle.
The incident was the first such armed seizure along the frontier in the conflict between Ukraine and Russia.
This week, Western monitors have witnessed another increase in the presence of Russian troops.
From 8 July until 16 July, there were 100 Russian positions on the border, nearly a tenfold increase on the number recorded from the previous month.
Reuters, the international news agency, says that on Tuesday alone, the monitors counted 2,350 military vehicles – including tanks, armoured personnel carriers and helicopters – within Ukraine’s borders.
Image copyright Reuters Image caption Ukraine says one of the attackers on Friday was a Russian soldier who handed himself over to Ukrainian border guards
While this is one of the highest numbers in recent times, it is low compared to the presence at checkpoints that NATO experts consider a border, according to the BBC’s James Reynolds.
Reuters reports that the monitors have counted 800 military vehicles in their last three count.
Some of the troops are seen, members of all ages, clutching gas masks. This image has stirred outrage in Ukraine and prompted President Petro Poroshenko to demand an investigation by the United Nations.
The International Committee of the Red Cross has also expressed concerns, saying the equipment was in violation of Ukraine’s laws.
However, Monitor Ukraine and the Vienna-based European Center for Constitutional and Human Rights both said they had seen no evidence of “war crimes” in the event.
‘Committed to improving border security’
The Red Cross, which says it did not inform Ukrainian authorities that it was carrying out the exercises near the border, has warned it is committed to “improving border security and ensuring that humanitarian aid is delivered unimpeded”.
It says it is prepared to continue the work in the east, where clashes continue.
Ukrainian officials have called the monitors “provocateurs” and “undeclared foreign fighters” and accuse Russia of arming the separatists.
In June, a UN Security Council committee met to discuss the tensions and asked for additional information about the border inspection teams.
The tension could intensify if the 200,000 internally displaced people return to their homes.
In some cases, according to humanitarian groups, they are returning to demolished buildings and burning villages.
BBC Africa editor Alan Johnston says a clash between the security forces and pro-Russian separatists that could spark an international conflict would be hugely embarrassing for Russia.
And Ukrainian military analyst Anatoly Iskanderov says if Russia decides to bolster the separatists further, the insurgency could reach Moscow in its most devastating form yet.
Ukraine has been locked in a conflict with Russia since a pro-Russian separatist uprising broke out in the east of the country in 2014.
More than 10,000 people have been killed in the fighting and more than a million refugees have been forced to leave their homes.
For more of our continuing coverage on the conflict, read our ground-breaking profile of the men behind Ukraine’s conflict