Andriy Lysenko, the spokesman for the Ukrainian government’s National Security and Defense Council, said the main obstacle to reaching and securing the wreckage of the plane was rebel resistance. “At the crash site of the Boeing 777, the terrorists have made new fighting positions,” he said. “They have amassed their heavy artillery, and they mined the approaches to this territory. This made impossible the work of international experts who began trying to carry out their duties and establish the reasons for the crash.”

Russia, however, said Ukraine and the United States were fueling the violence in the area. “It’s not Russia but the Kiev regime and its overseas sponsors who are to blame for the growing number of victims among the civilian population of the eastern regions,” the Russian Foreign Ministry said in a statement.

It said the expanded sanctions announced on Tuesday would make peace harder to achieve. Imposing them, it said, can only result in “further aggravation of Russian-American relations and the creation of an extremely unfavorable background in international affairs, where collaboration between our countries often plays a decisive role.”

Dmitri O. Rogozin, the Russian deputy prime minister in charge of the military industry, suggested that the United States had imposed new restrictions on arms and technology sales to Russia out of fear of its growing might. He said on Twitter that the measures were “a sign that Russian military shipbuilding is becoming a problem for the enemies of Russia.”

But Ukrainian leaders saw in the sanctions announcement a new unity and resolve in the West to rein in Russia. “What even a month or a month and a half ago was considered impossible has become a reality,” Mr. Chaly said. “We hope by joint action of the international community, we will be able to convince our neighbor to stop the aggression against us.”