Loud explosions have been heard on the eastern outskirts of the Ukrainian city of Mariupol, despite a fragile ceasefire being put in place.

Numerous explosions were heard and a thick smoke was visible on the horizon. A checkpoint held by Ukraine loyalists seemed to be on fire, according to journalists close to the scene.

Three Ukrainian tanks were seen heading towards the checkpoint but then turned round. Dozens of cars were seen heading away from where the explosions took place.

The Ukrainian president Petro Poroshenko and his Russian counterpart Vladimir Putin agreed on Saturday in a phone call that the truce “was generally being observed”.

Earlier, the cities of Donetsk and Mariupol, where the fighting had been most intense over the past week, were quiet, city authorities and local media said.

However, reports said that fighting continued during the night at Donetsk airport, which has been held by government forces since May.

Separatist commanders said on Saturday that the ceasefire took hold only at 8pm on Friday, two hours after it came into force, the Interfax news agency reported.

The ceasefire is part of a 12-point agreement signed by separatist leaders and the Ukrainian government in the Belarusian capital Minsk.

The agreement also envisages the exchange of about 1000 prisoners that are being held by both sides.

Ukrainian President Petro Poroshenko has said that the exchange could start as early as Saturday.

A Russian shipment of humanitarian aid is expected to head to eastern Ukraine by railway on Saturday.

US President Barack Obama said he was hopeful the ceasefire would hold but unsure the rebels would follow through and that Russia would stop violating Ukraine’s sovereignty and territorial integrity.

“It has to be tested,” Obama said on Friday at the closing of a two-day NATO summit in Wales.

Both the US and the European Union have prepared even tougher sanctions on Moscow, and Obama stressed that the most effective way to ensure the ceasefire’s success was to move ahead with those measures and maintain pressure on Russia.

According to an EU diplomat, these new measures would target Russia’s access to capital markets and trade in arms and defence technology, dual-use goods and sensitive technologies.

The new sanctions were given preliminary approval on Friday night and could be implemented as early as Tuesday.

“If certain processes get underway, we are prepared to suspend sanctions” against Russia, German Chancellor Angela Merkel said.

In a statement published online on Saturday, Russia’s foreign ministry condemned further EU sanctions and promised that “there will undoubtedly be a reaction from our side” to any new measures.

In August, Russia passed a sweeping ban on meat, fruit, vegetables and dairy product imports from the EU, the US, Australia, Canada and Norway who imposed sanctions on Russia.

Moscow criticised the European Union for drawing up the tougher sanctions despite the truce agreed in Minsk.

But by announcing new sanctions, the EU’s leadership “is practically sending a signal of direct support to the ‘party of war’ in Kiev, which is not happy with the results of the Minsk meeting,” Russia’s foreign ministry said.

“Instead of feverishly searching for ways to hurt the economies of its own countries and Russia, the European Union would do better to work on supporting the economic revival of the Donbass region” of eastern Ukraine, it added.