It is not the right moment for any United Nations peacekeeping troops to be sent to Ukraine, U.N. Secretary General Ban Ki-moon was quoted as saying on Tuesday.

"At this moment, it doesn't seem very practical to send troops," Ban told Mexican newspaper Reforma, according to the Spanish text of an interview.

Ban, who was in Mexico City to attend a conference on international development, was speaking as Ukrainian forces launched a "special operation" against separatist militia in the Russian-speaking east of the country.

Tensions have flared between Moscow and the West in Ukraine since Russia's annexation of the Crimean region last month after the overthrow of Ukraine's Moscow-backed president, Viktor Yanukovich.

Ban told the newspaper that any decision to send troops to Ukraine would need to be agreed by the U.N. Security Council, of which Russia is a permanent member.

"Unless we have a clear mandate and authorization from the Security Council, I can't take any action," Ban said.

Earlier on Tuesday, the Kremlin said Russian President Vladimir Putin told Ban that the United Nations and the international community should condemn the use of force by Ukrainian authorities in eastern Ukraine.