PARIS — France demanded formal guarantees from Iran on Thursday over what it described as the "serious intimidation" of its diplomats in Tehran, a day after summoning Iran's charge d'affaires to the foreign ministry. The French foreign ministry said the pressure on embassy staff was unacceptable.

The dispute deepens a long-running crisis between Paris and Tehran. France has for months demanded the release of two of its nationals, Cecile Kohler and Jacques Paris, held in Iran on espionage-related charges that Paris rejects as baseless. French officials have described the pair as state hostages.

France's foreign ministry spokesman said the intimidation of accredited diplomats violated the Vienna Convention on Diplomatic Relations. The ministry said it had raised the matter directly with Iranian authorities and would coordinate a response with European partners.

Tehran has previously accused France and other European governments of hostile actions, and relations have soured further since the June conflict between Israel and Iran. Iranian officials have signalled they may reduce cooperation with Western states, including on consular matters.

The standoff risks further isolating Iran diplomatically as European governments weigh reactivating United Nations sanctions. France, Britain and Germany have warned Tehran that the "snapback" mechanism under the 2015 nuclear deal remains available before its October expiry.