Mr. Kerry did not mince words in a news conference Thursday in Kiev, laying blame for the renewed violence in eastern Ukraine at the door of the Kremlin: “We talked about the largest threat that Ukraine faces today, and that is Russia’s continued aggression in the east.” He called on Moscow and the separatists to “support and honor the commitment they made to implement a cease-fire, to pull back heavy weapons and troops” and to respect Ukraine’s international border with Russia.

The accelerated Western diplomatic efforts came as the Obama administration was considering whether to send basic weapons like antitank missiles, battlefield radars, reconnaissance drones and other arms to help Ukraine’s beleaguered forces stave off attacks by the separatists and build pressure on Moscow to seek a political settlement.

Mr. Obama’s top national security officials — the so-called Principals Committee — met on Wednesday to discuss the matter, though no decision was made.

The head of the Ukrainian security service, Valentyn Nalyvaichenko, said at a news conference on Wednesday that the accused spy, Lt. Col. Mykhailo Chornobai, had been at the center of an espionage ring in the capital and had passed military secrets directly to an agent of the separatist Donetsk People’s Republic, including the locations of volunteer regiments that were then used to pinpoint artillery attacks.

Dmytro Tymchuk, a military officer and member of Parliament, said that Colonel Chornobai was among about 300 people working in the military sphere who had been arrested since the start of the conflict.

The arrest further deepened mistrust of the leadership in Kiev that is already pervasive among the poorly equipped rank-and-file soldiers and midlevel commanders fighting on the front line. And it reinforced a view prevalent on the battlefield that the military leadership cannot be trusted to manage any weapons delivered by Western allies because of their ties to the Russian military and security service, the F.S.B.