Bulgaria has denied use of its airspace for Russian aid flights to Syria due to doubts about the nature of the cargo Moscow sends to the war-torn country. It comes on the heels of a similar Washington request to Greece.

The move has been confirmed by the Bulgarian Foreign Ministry.

Bulgaria "refused to issue permits for flights through Bulgarian airspace of Russian military transport aircraft en route from the Russian Federation to Syria in the period between September 1 and 24, 2015," Sofia's statement said.

"We have information which gives us grounds to doubt the correctness of the information stated in the request about the purpose of the flights and the cargo transported," ministry spokeswoman Betina Joteva said, citing the reason behind the refusal.

Earlier, the ministry official told TASS news agency that Sofia had doubts whether the nature of the goods intended to be carried complied with that stated in the request. The ministry "made the decision independently," Joteva added, giving no further details.

A Russian buildup in Syria? The propaganda machine strikes again (Op-Edge by @27khv) http://t.co/Tuyos3yAPRpic.twitter.com/7h5Ar2mzPH — RT (@RT_com) September 8, 2015

Such an “unfriendly” move will not be forgotten, and Russia will find an “adequate” response, Nikolay Levichev, a Russian politician and lawmaker specializing in international affairs, told TASS.

“With such a decision – whatever has forced it – the Bulgarian leadership has joined the camp of adversaries of the existing system of international relations,” Levichev said, adding that closing airspace to planes carrying humanitarian aid to war-torn Syria was “an unhuman and clearly short-sighted act.”

Alternative routes for the Russian planes flying to Syria have already been found, the lawmaker added.

MORE: Greek foreign ministry confirms receipt of request from US to block airspace http://t.co/GMWwtWQMDbpic.twitter.com/IiKT7GUxDH — RT (@RT_com) September 7, 2015

Earlier Tuesday, Greece’s Kathimerini newspaper reported that Bulgaria had denied Russia use of its airspace, meaning that two Russian planes would have to use Iranian airspace.

The news from Bulgaria came a day after the Greek Foreign Ministry confirmed it had received a request from Washington asking that Russia be denied use of Greek airspace for aid flights to Syria.

Russia has repeatedly delivered humanitarian aid to Syria. In August this year, Moscow sent over 20 tons of humanitarian aid, including tinned meat, fish and milk, sugar and blankets to the city of Latakia. In May, the city received 21.5 tons of supplies from Russia.

Russian planes also transported hundreds of people, both Russians and foreigners, from the conflict zone.

War-torn Yemen, Iraq and Ukraine are also among the countries to have received humanitarian aid from Russia this year.

In 2014 Russia delivered 11,000 tons of humanitarian aid to 24 countries, including Syria, Afghanistan, Yemen and Iraq, the Emergencies Ministry told RIA Novosti. Apart from that, 13,400 tons of aid were delivered to Donbass in eastern Ukraine, where the civil war has already killed almost 8,000 people.