06:50
Analysis: Stakes have suddenly got higher after nuclear move signals shift in Trump-Putin relationship

By Ivor Bennett, Moscow correspondent

Normally it's Moscow rattling the nuclear sabres, but this time it's Washington in what marks a dramatic escalation in Donald Trump's war of words with the former Russian president Dmitry Medvedev.

More importantly, it appears to signal a significant deterioration in his relationship with Vladimir Putin.

The US president's patience with the Kremlin was already at its thinnest earlier this week, when he shrank his deadline for progress towards a peace deal from 50 days to 10.

But Russia's lack of outward concern with this stricter ultimatum - which has swung from dismissive to (in Medvedev's case) insulting - seems to have flicked a switch.

For this is the first time Trump's pressure on Moscow has amounted to anything more than words.

We don't know where the subs are, or how far they had to move to get closer to Russia, but it's an act that sits several rungs higher than the usual verbal threats to impose sanctions.

How will Russia respond? I'm not sure Vladimir Putin has ever caved to an ultimatum and I doubt he'll start now.

But I don't think he'll want the situation to deteriorate further. So I suspect he'll make another offer to the US, that's dressed up as a concession, but in reality may prove to be anything but.

It's a tactic that's worked before, but the stakes have suddenly got higher.