As it moves to transform U.S. relations with Russia, the Trump administration is talking with Moscow about readmitting potentially scores of Russian diplomats into the United States after years of expulsions.

But the good-will gesture, which would be reciprocated by Moscow, could be a kind of Trojan horse, experts and diplomats warn, as the Kremlin is likely to dispatch spies posing as diplomats to restore its diminished espionage capabilities within the United States.

U.S. and Russian officials met in Istanbul last month to discuss returning more diplomats to each other’s countries after years of tit-for-tat expulsions and the shuttering of diplomatic facilities. The midlevel talks, part of a rapid rapprochement between the Kremlin and the White House under President Trump, took place at the U.S. consul’s residence.

Days earlier in Riyadh, a U.S. delegation headed by Secretary of State Marco Rubio and top Russian officials agreed “to ensure that our diplomatic missions can function,” as Mr. Rubio told reporters.

Both sides say the move could pave the way for a broader peace agreement to end the war in Ukraine.

An agreement to normalize diplomatic operations might also enable the United States to conduct espionage activities of its own: Washington has long placed spies in U.S. embassies and consulates in Russia. But experts say that even if a deal expands both diplomatic contingents in comparable numbers, any Russian spies would enjoy an advantage, working in a more open society in the United States.

The renewed access, combined with Mr. Trump’s courtship of President Vladimir V. Putin of Russia, could spell opportunity for the Kremlin’s espionage apparatus at a time when Moscow’s operations against the West have grown more brazen, according to intelligence experts and former officials.