When Ukraine successfully targeted Russia's precious warship last month with anti-ship missiles, they had help from the United States.

According to foreign media, the Telegraph reports, Ukrainian forces, after spotting a Russian warship in the Black Sea, called their American contacts to confirm that it was in fact Moscow, sources familiar with the events told CNN.

And the United States responded that it was, and provided information about its location.

It is unclear whether the US knew Ukraine would take steps to strike the ship, however, and the US was not involved in that decision, sources said.

The ship sank after being hit by two Ukrainian missiles on April 14, giving a major blow to the Russian army.

The episode, CNN further writes, reflects the increasingly inclined stance of Joe Biden's administration when it comes to intelligence sharing with Ukraine, part of a broader policy shift towards Ukraine's help in defeating Russia resolutely in the field. of battle and significantly weaken its army.

But it also raises the question of what are the red lines of the US and Russia when it comes to US military support for Ukraine.

For months, the United States has provided Ukrainian forces with information on the movements of Russian troops inside Ukraine, including wiretapped communications about Russian military planning.

The US also provides Ukraine with maritime awareness information to allow them to better understand the threat posed by Russian ships in the Black Sea, many of which are launching missiles into Ukrainian territory.

There are also clear boundaries, however, on what the US will share, multiple sources have told CNN.

For example, the US has so far refused to provide Ukraine with information about possible targets within Russia itself.

And while US intelligence on the movements of Russian troops inside Ukraine may include details about vehicles and types of personnel in a particular country, the US has not provided Ukraine with information on the whereabouts of Russian special military leaders, officials said. .