Blinken, widely reported to be pressing President Joe Biden to ease rules that prevent Ukraine from striking targets on Russian soil, stopped short of announcing a change but hinted he was receptive the calls.

Since Russia's invasion, "as the conditions have changed, as the battlefield has changed, as what Russia does has changed in terms of how it's pursuing its aggression (and) escalation, we've adapted and adjusted," Blinken told reporters on a visit to Moldova on the eve of NATO talks in Prague.

Asked if he was supporting a change, Blinken repeated: "At every step along the way we've adapted and adjusted as necessary. And so that's exactly what we'll do going forward."

"We're always listening, we're always learning and we're always making determinations about what's necessary to make sure that Ukraine can effectively continue to defend itself," he said.

Blinken, who visited Ukraine earlier this month, voiced confidence in Kyiv as its forces began to receive US weapons that were stalled by partisan fighting in Washington.

He said that Russian battlefield victories still showed a failure in President Vladimir Putin's earlier aim of seizing the city of Kharkiv.

"If you project out over the next period of time, we are on the path to help ensure that Ukraine is a success -- a country that stands strongly on its own feet militarily, economically, democratically. And that's the ultimate rebuke to Mr. Putin," Blinken said.