Russia scrapped plans Wednesday to refuel Syria-bound warships in a Spanish port after Madrid came under pressure to refuse access to a flotilla that may be used to help attacks on Aleppo.

Spain's foreign ministry said three Russian ships had been due to take on fuel and supplies at the port of Ceuta, a Spanish territory on the north coast of Africa across the sea from Gibraltar.

These were believed to be part of a wider fleet led by aircraft carrier Admiral Kuznetsov.

There has been concern that the ships could take part in air strikes in Syria, where Russia has been conducting a bombing campaign for the past year in support of President Bashar al-Assad and has deployed a naval contingent to back up its operation.

- Used for Aleppo? -

"The Russian embassy in Madrid... told us that it is withdrawing its demand for permission to stop over for the boats, which means that the stop-overs have been cancelled," the Spanish foreign ministry said in a statement.

Moscow's decision comes against a backdrop of increasing tensions between Russia and the West over the war in Syria, as well as the conflict in Ukraine.

Just last week, the European Union's 28 leaders -- including Spanish acting Prime Minister Mariano Rajoy -- signed a draft statement condemning "the attacks by the Syrian regime and its allies, notably Russia, on civilians in Aleppo".

Russian ships have for years made stops in Spanish ports, with permission granted on a case-by-case basis, taking into account the safety of local residents and the environment, Spain's foreign ministry said.

"(But) faced with information that materialised about the possibility that these ships would help support military actions in the Syrian city of Aleppo, the foreign ministry asked the Russian embassy in Madrid for clarification on this information," it said.

Following this, Russia cancelled the planned stop-over, it added.

News of the stop-over prompted criticism Tuesday.

The foreign ministry said Spain had in September given Russia permission to refuel from October 28 to November 2.

But Mike Walliker, the commander of British forces in Gibraltar, said Wednesday evening that the fleet had already sailed through the Strait of Gibraltar into the Mediterranean.

The first group, comprised of escort vessels, entered at around 4:00 am local time (0200 GMT), he told AFP.

The second -- which included the aircraft carrier -- sailed through several hours later, passing Gibraltar just before sunrise, he added.

- 'Scandalous' -

NATO Secretary-General Jens Stoltenberg had expressed concern that the fleet could take part in Syria air strikes.

"It's a decision which has been taken by individual allies, whether they provide fuelling and supplies to Russian ships," he told reporters in Brussels.

"But this time I have conveyed a very clear message that we are concerned about the potential of this carrier group to increase attacks in Aleppo," he added.

"All allies are aware of our concerns, they share our concerns about Russian airstrikes against Aleppo."

A British government spokesman said that London "has previously expressed concerns to the Spanish government about its hospitality to the Russian navy when we have concerns about Russia's military activity."

Guy Verhofstadt, former Belgian premier and liberal leader in the EU parliament, said in a Twitter message it was "scandalous" that Spain, a member of NATO and the EU, would allow the Russian Kuznetsov fleet "to refuel and receive technical assistance on Spanish territory."

At least 57 Russian navy ships stopped in Ceuta between 2011 -- when Moscow started to regularly use the port facilities there -- and August 2015, according to conservative US think tank Heritage Foundation.

The foundation blasted Spain for allowing Russia to use the port even after Russia invaded and annexed Crimea in March 2014.

It said Greece, another NATO member, as well as Malta, which is part of the EU but not NATO, also allow Russian navy ships to use their ports.

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ADMIRAL GROUP