By Humeyra Pamuk WASHINGTON (Reuters) -U.S. Secretary of State Antony Blinken on Friday held a telephone conversation with Russian Foreign Minister Sergei Lavrov and pressed the Kremlin to accept the ‘substantial proposal’ that Washington has put forward to secure the release of two Americans detained in Russia. In their first such conversation since Russia’s invasion of Ukraine on Feb. 24, the two top diplomats looked to stick to their existing positions, according to the readout of the call from Blinken and the Russian foreign ministry. “We had a frank and direct conversation,” Blinken told …