LONDON: Britain summoned Iran‘s ambassador on Monday after charges were filed against three Iranian nationals accused of acting as spies. The move came the day after Iran summoned a British diplomat to protest what it described as the “illegal and unjustified” detention of an Iranian in the UK, Iran’s state-run news agency reported. The tit-for-tat comes as ties between Tehran and London deteriorate. Iran called the detention of its citizen, who was not identified, a violation of international law and expressed strong dissatisfaction over what it characterized as politically motivated judicial actions, Iran’s state-run news agency reported. The ministry warned the UK against further “unconstructive behavior” that could damage diplomatic relations, IRNA said. Three men accused of being Iranian spies faced charges in a London court Saturday that they conducted surveillance on and plotted violence against UK -based journalists for an Iranian news outlet. On Monday, Britain summoned Ambassador Seyed Ali Mousavi to the Foreign Office in response to the criminal case. “The UK government is clear that protecting national security remains our top priority and Iran must be held accountable for its actions,” the ministry said in a statement. When the three men were arrested in the UK two weeks ago, police also took four other Iranian nationals into custody on suspicion of preparing a terrorist act in a separate investigation. The four were released from custody Saturday, though counterterror police said that the investigation continues.