British Foreign Secretary Philip Hammond said that Russia potentially poses “the single greatest threat” to U.K.’s security, and accused President Vladimir Putin of “subverting” the system of rules that “keeps the peace between nations,” according to media reports. Hammond’s comments came during a speech at the Royal United Services Institute in London on Tuesday.

“We are now faced with a Russian leader bent not on joining the international rules-based system which keeps the peace between nations … we are in familiar territory for anyone over the age of about 50, with Russia's aggressive behavior a stark reminder that it has the potential to pose the single greatest threat to our security,” Hammond reportedly said, adding that the Russian military’s increasingly aggressive stance in Ukraine, and Russian aircraft activity over the sovereign airspace of NATO nations, is a “significant cause for concern.”

“Hence, continuing to gather intelligence on Russia’s capabilities and intentions will remain a vital part of intelligence effort for the foreseeable future,” Hammond added, according to media reports.

In his speech, Hammond also raised the issue of home-grown jihadists, including “Jihadi John,” who was recently identified as Kuwait-born Briton named Mohammed Emwazi. After his identity was revealed, CAGE -- a London-based advocacy group for “communities impacted by the War on Terror” -- reportedly said that U.K.’s MI5 had played a role in Emwazi’s radicalization.

“The exposure of the alleged identity of one of the most murderous Isil terrorists over the last few weeks has seen some seeking to excuse the terrorists and point the finger of blame at the agencies themselves,” Hammond said. “We are absolutely clear; the responsibility for acts of terror rests with those who commit them … but a huge burden of responsibility also lies with those who act as apologists for them.”