Russian president Vladimir Putin has called for improvement of Moscow's secret services to tackle "modern challenges and threats" amid his country's standoff with the West over the Ukrainian crisis.

The call came in a letter by Putin, himself a former KGB agent, to veterans and current operatives of Russia's security services on the day Moscow traditionally honours them. The key tasks for Russia's secret operatives were to fight global terrorism and "any attempts of foreign special services to deal a blow to Russia [and] her political and economic interests", he said.