Two major European airlines said Thursday that they would suspend service to Tehran next month, a double-punch that underscored the power of reimposed American sanctions on Iran and the limited abilities of others to sidestep them.

The suspensions, by British Airways and Air France, mean at least three large European carriers, which once held out great promise for their Iran business under the now-threatened nuclear agreement, will quit flying to and from the country in September. KLM, the Dutch sister airline of Air France, announced a similar suspension last month.

The moves seemed bound to deepen Iran’s sense of economic isolation, which has worsened considerably in the nearly four months since President Trump scrapped American participation in the nuclear agreement negotiated by the administration of his predecessor, Barack Obama.

Mr. Trump and his aides have warned foreign businesses to steer clear of Iran to avoid reimposed American sanctions, which take full effect in November.