PARIS (Reuters) - France summoned Iran's ambassador on Friday to demand the release of two French citizens being held in Iranian jails after one of them, Franco-Iranian researcher Fariba Adelkhah, began a hunger strike.

"As the president and foreign minister have underlined on multiple occasions, their imprisonment is intolerable," the French Foreign Ministry said in a daily online press briefing.

Iran has stepped up detentions of foreign and dual nationals on spying and security charges during a protracted standoff with Western powers since the United States withdrew from an international agreement to curb Iranian nuclear activities.

Tehran has so far rebuffed French President Emmanuel Macron's demands that it release Adelkhah and Roland Marchal, her Paris Institute of Political Studies (Sciences Po) colleague. The two academics were arrested in June.

Both have been refused consular access throughout their detention, the French government says.

The Iranian envoy was received by one of France's most senior Foreign Ministry officials.

Adelkhah and another arrested foreign academic, British-Australian Kylie Moore-Gilbert, said in a letter dated Dec. 24 they had begun refusing food and water at the Evin prison in Tehran.