Turkey, South Africa and Russia have reacted angrily to the British government demanding an explanation for the revelations that their politicians and senior officials were spied on and bugged during the 2009 G20 summit in London.

The foreign ministry in Ankara said it was unacceptable that the British government had intercepted phonecalls and monitored the computers of Turkey's finance minister as well as up to 15 others from his visiting delegation. If confirmed, the eavesdropping operation on a Nato ally was "scandalous", it added.

The ministry summoned the UK's ambassador to Ankara to hear Turkey's furious reaction in person. A spokesman at the foreign ministry read out an official statement saying: "The allegations in the Guardian are very worrying … If these allegations are true, this is going to be scandalous for the UK. At a time when international co-operation depends on mutual trust, respect and transparency, such behaviour by an allied country is unacceptable."

The Guardian revealed that the UK secret wiretapping agency, GCHQ, targeted Mehmet Şimşek, the Turkish finance minister and a former Merrill banker, during a G20 economics meeting hosted in London in September 2009. It also considered monitoring the communications of 15 named members of his staff and of Turkey's central bank. It is not clear which if any of the staff members was ultimately placed under surveillance.

The goal was to collect information about the Turkish position on the reform of the global financial infrastructure in the wake of the world banking crisis.

The revelations come at a fraught time for Turkish-British relations. The country's embattled prime minister, Recep Tayyip Erdoğan, has blamed the international media, and in particular the BBC, for fomenting violent unrest and protests against his rule. Erdoğan has spoken repeatedly of an "international conspiracy". News that his finance minister really was the victim of a British surveillance operation will strengthen his view.

The South African foreign ministry, which was a target of a GCHQ hacking operation launched in 2005, also voiced its concern. A ministry statement said: "We do not yet have the full benefit of details reported on but in principle we would condemn the abuse of privacy and basic human rights particularly if it emanates from those who claim to be democrats."

The statement added: "We have solid, strong and cordial relations with the United Kingdom and would call on their government to investigate this matter fully with a view to take strong and visible action against any perpetrators."

GCHQ documents seen by The Guardian showed the British hacking operation was designed to get information from foreign ministry computer networks on briefings given to ministers for G20 meetings and also G8 summits attended by South Africa as an observer.

In Moscow Russian officials said the Guardian revelation that US spies had intercepted top-secret communications of Dmitry Medvedev at a G20 summit in London in April 2009 would further harm the struggling US-Russia relationship and cast a shadow over the G8 summit in Northern Ireland on Monday and Tuesday.

Details of the spying, set out in a briefing prepared by the National Security Agency (NSA), were leaked by the whistleblower Edward Snowden and revealed by the Guardian late on Sunday. Documents show that US spies based in Britain spied on Medvedev, then the Russian president and now prime minister.

Medvedev's spokeswoman, Natalya Timakova, declined to comment. But speaking to state-run media, senior Russian officials said the revelations had deepened mistrust between the US and Russia, whose relations have already sunk to a post-cold-war low following a brief and largely unsuccessful "reset" during Medvedev's four-year reign in the Kremlin.

Igor Morozov, a senator in Russia's Federation Council, the upper house of parliament, suggested that the Obama administration's attempts to improve relations were clearly insincere: "2009 was the year the Russian-American 'reset' was announced. At the same time US special services were listening to Dmitry Medvedev's phonecalls."

He added: "In this situation, how can we trust today's announcements by Barack Obama that he wants a new 'reset'? Won't the US special services now start spying on Vladimir Putin, rather than correcting their actions?" he told RIA-Novosti, a state-owned news agency. "This isn't just an act of inhospitality, but a fact that can seriously complicate international relations," he said. "Big doubts about Obama's sincerity appear."

The revelations were the lead story on Russia Today, the Kremlin's international propaganda TV channel.

It also featured elsewhere. Domestic NTV, owned by the state gas giant Gazprom and run by the Kremlin, commented: "The spy scandal can cast a cloud over the G8 summit opening today."

Former top-ranking Russian spies, meanwhile, suggested the behaviour by their US and UK counterparts amounted to bad form. "From a technical point of view, spying on those negotiating on the territory of a country doesn't present any great difficulties," Nikolai Kovalev, the former head of the FSB, Russia's powerful domestic spy agency, pointed out. Kovalev added however: "To avoid diplomatic and international scandal security agencies are forbidden from doing this. And usually they don't do it."

Russia and the US have been plagued by spy scandals for years – just last month Russia expelled a US embassy employee in Moscow charged with being a CIA spy, and in 2010 the US busted a ring of Russian sleeper spies posted throughout the country.

But news of the high-level spying in a third country comes at a time when Putin has made whipping up anti-Americanism a top priority. The two countries remain at odds over Syria, and Putin has repeatedly accused the US state department of funding and directing opposition to him at home.

Alexey Pushkov, the head of the Duma's international affairs committee and one of the loudest anti-American voices in the Russian government, took to Twitter to write: "Scandal! In 2009 at the G20, US and UK special services listened to Medvedev's telephone calls. The US denies it, but we can't believe that. That's complete fraud."

Others were more sanguine. Viktor Ozerov, head of the Federation Council's defence and security committee, said: "Russia shouldn't take this [spying] for granted, but shouldn't dramatise the situation either. Intelligence agencies exist to spy not only on private citizens but on top government leaders too."