Ukraine has denounced a visit by Russian Prime Minister Dmitry Medvedev to Crimea, describing it as a "crude violation of the rules of international behavior."
Foreign Ministry spokesman Evhen Perebiynis told journalists in Kyiv that Ukraine had sent a note to the Russian Federation expressing "a categorical protest" over the visit.
Medvedev said in Simferopol earlier on March 31 that Moscow will make the region a special economic zone in order to attract investors.
At the head of a delegation with top Russian ministry officials, Medvedev said Moscow's goal is to enable Crimea "to generate sufficient income for its own development."
He vowed to raise the level of salaries for municipal employees and pensions to average Russian levels and to modernize the region's hospitals, which he said were outdated.
"As I already said, the people of Crimea must only benefit from joining Russia," Medvedev said. "It is our duty together to ensure that."
Medvedev toured Simferopol amid a heavy police presence and is also due to visit Russia's Black Sea fleet base in Sevastopol.
The Russian prime minister's visit to Crimea is the highest-level official visit from Moscow since Russia seized the territory from Ukraine.
It comes just hours after U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov met in Paris late on March 30 to discuss the crisis in Ukraine.
After the meeting, Kerry said he and Lavrov agreed that diplomacy is the best way to defuse the Ukraine crisis.
Kerry said there will not be any decisions on Ukraine without the involvement of Ukrainians.
"The United States is consulting with Ukraine at every step of this process and we will not accept a path forward where the legitimate government of Ukraine is not at the table," he said. "This principle is clear: no decisions about Ukraine without Ukraine."
Kerry also said he raised Washington's "strong concerns" about the massing of Russian forces on Ukraine's eastern border.
"Any real progress in Ukraine must include a pullback of the very large Russian force that is currently massing along Ukraine's borders," he said. "And tonight, I raised with the [Russian] foreign minister our strong concern about these forces. We believe that these forces are creating a climate of fear and intimidation in Ukraine."
Lavrov said he and Kerry agreed to work with the Ukrainian government to improve rights for Russian-speaking Ukrainians and to disarm what he called "irregular forces and provocateurs."
At a separate briefing, Lavrov said Ukraine couldn't function as a unified state and instead should be a loose federation of regions that choose their own economic model, language, and religion.
The Kerry-Lavrov meeting in Paris comes before NATO foreign ministers meet in Brussels on April 1-2 for talks likely to focus on Ukraine and Russia's actions.
Russia's annexation of Crimea is not internationally recognized and has been called illegal by the United States, the European Union, and the United Nations General Assembly.
The United States and the European Union have imposed two rounds of sanctions on Russia, including visa bans and asset freezes for some of President Vladimir Putin's inner circle, to punish Moscow over its seizure of Crimea.
Western leaders say further sanctions, some targeting key sectors of the Russian economy including its natural gas and oil industry, could be imposed if Moscow continues to destabilize Ukraine.
With reporting by Reuters and Interfax