Milley, speaking in London on Tuesday ahead of stops in Finland and Sweden as they bid to join NATO, noted Black Sea shipping lanes were blocked by mines and the Russian navy.

This has confined dozens of container ships to Ukrainian ports, including the main hub of Odessa, preventing global grain supplies being delivered and prompting fears of a worldwide food crisis.

"In order to open up those sea lanes, that would require a very significant military effort by some nation or group of nations," Milley told reporters travelling with him through Europe.

"Options are always looked at... I wouldn't ever rule anything out, or never rule anything in, but I would tell you that that would be a high-risk military operation that would require a significant level of effort."

Milley -- who last month spoke by phone with his Russian counterpart General Valery Gerasimov in their first discussion since Russian forces invaded Ukraine in February -- added any decision over the blockade would be "policy and political choices".

While in London, Milley, the chairman of the Joint Chiefs of Staff, will attend a meeting Wednesday of the "Five Eyes" intelligence alliance comprising the United States, Britain, Australia, Canada, and New Zealand.

His attendance was "to continue to assure our allies and partners that the United States is here and that we are a good friend, a good ally, a good partner," he said.

The US general will also represent Washington, alongside other American officials, on Thursday as four days of celebrations get underway for Queen Elizabeth II's Platinum Jubilee.

On Friday, he will head to Helsinki for talks with President Sauli Niinisto and defence minister Antti Kaikkonen, after Finland -- which shares a long border with Russia -- applied to join NATO along with Sweden.

The following day, Milley will watch joint military manoeuvres between the Western alliance and the Swedish navy, alongside that country's defence minister Peter Hultqvist.

He will also have an audience with Swedish King Carl XVI Gustaf.

The final leg of the tour will take Milley to France, where he will participate in several ceremonies marking the WWII Normandy landings, and then on to a June 8 meeting of NATO members' chiefs of staff devoted to Ukraine.