Trump sent second letter to Erdogan threatening sanctions over S-400s
Letter set out US conditions for resolving crisis over Russian-made missiles ahead of White House meeting, but Turkish president was not convinced, sources say
Turkey may purchase Russian warplanes, says Erdogan
Erdogan: "Turkey did not ask anyone permission when buying the Russian S-400 missile system nor will not do so if it decides to acquire Russian fighter jets"
Turkey says delivery of second Russian S-400 batch may be delayed
And Washington still hopes to persuade its ally to "walk away" from the Russian systems...