DONETSK, Ukraine — Ukrainian forces on Monday zeroed in on rebel strongholds as the government welcomed an international humanitarian relief mission into the rebellious east involving Russia, the United States, and the European Union.
The mission will be conducted under the auspices of the International Committee of the Red Cross. The organization said in a statement it is ready to facilitate the operation with the involvement of all sides concerned following a Russian initiative to provide humanitarian assistance to people in eastern Ukraine.
It wasn’t clear when the deliveries would start.
‘‘The practical details of this operation need to be clarified before this initiative can move forward,’’ said Laurent Corbaz, the ICRC’s head of operations for Europe and Central Asia.
Moscow had long urged Kiev to allow the aid delivery, but Ukraine and the West previously had opposed the move, fearing it could serve as a pretext for sending Russian troops into rebel-held territory. Ukraine and the West have accused Moscow of arming and supporting the rebels fighting government troops in the east, a charge that the Kremlin has denied.
The Red Cross said it has shared a document with Ukrainian and Russian authorities that stipulates all parties must guarantee the security of its staff during the operation and respect the organization’s neutrality.
The aid mission was announced after a conversation between President Obama and President Petro Poroshenko of Ukraine Monday.
The White House said that Obama and Poroshenko agreed that ‘‘any Russian intervention in Ukraine without the formal, express consent and authorization of the Ukraine government would be unacceptable and a violation of international law.’’
Shortly before that, Russia had declared that it was dispatching a humanitarian convoy into Ukraine in cooperation with the Red Cross without giving any details. President Vladimir Putin’s spokesman, Dmitry Peskov, later was quoted by Russian news wires as saying that the convoy wouldn’t involve military personnel.
Officials in Kiev took pains to specify Monday that the Ukrainian government was behind the humanitarian convoy initiative and that Moscow was only one of several countries involved.
‘‘Apart from deliveries provided by Ukraine, the mission will feature an international component, including aid provided to the International Committee of the Red Cross by the United States, the EU, as well as Russia,’’ Ukraine’s Foreign Ministry said.
The ministry also laid out specific conditions for the aid shipment, saying it should only pass through checkpoints controlled by the Ukrainian government. At least 60 miles of the long Russian-Ukrainian border is currently in rebel hands.
It said that the aid will be distributed by the Ukrainian authorities in the Luhansk region, one of the two mostly Russian-speaking rebel provinces.
Some of the heaviest impact on civilians has been seen in the rebel-held provincial capital Luhansk, which had a pre-war population of 420,000. City authorities said Monday the 250,000 residents remaining have had no electricity or water supplies for nine days.
Several Western diplomats and military officials said they were wary of Russia’s intentions. The secretary-general of NATO said there was a “high probability” of a Russian attack and Ukraine raised its estimate of Russian troop strength on the border.
NATO has said in the past that Russia had amassed about 20,000 troops along the border, enough to begin a ground invasion. But on Monday, a Ukrainian military spokesman, Andriy Lysenko, said the numbers had risen alarmingly, to around 45,000 troops. Those figures could not be independently verified.
In the last week, Ukrainian government forces have been closing on the few remaining rebel strongholds, including Donetsk, the largest rebel-held city.