LONDON, ENGLAND — While a Black Sea route for Ukraine’s grain through Turkey’s Bosphorus straight remains the best option, British Prime Minister Boris Johnson said alternative routes, including Europe’s Danube river, will need to be considered, Reuters reported.

Ukraine has been shipping grain by land because of the Russian blockade that started when Russia invaded Ukraine in February. Ukraine is among the world's top grain exporters, and it has some 30 million tonnes of grain stored in Ukrainian-held territory that it is trying to export via road, river and rail. Ukraine can typically ship 5 million tonnes of grain per month through its seaports. Ukraine has asked for help to break the blockade.

"The Turks are absolutely indispensable to solving this. They're doing their very best ... It does depend on the Russians agreeing to allow that grain to get out," Johnson told parliament, according to Reuters. "We will increasingly have to look at alternative means of moving that grain from Ukraine if we cannot use the sea route, if you can't use the Bosphorus."

Johnson told parliament alternatives such as railways or the Danube would be able to move grain out of Ukraine, but in much smaller quantities than could be moved by large convoys through the Black Sea. He said there were solutions that did not involve the use of British or other naval warships to facilitate safe transport.

"We are looking at all the possible options,” Johnson said.

Meanwhile, Reuters reported that Turkey had halted a Russian-flagged cargo ship off its Black Sea coast and is investigating a Ukrainian claim that it was carrying stolen grain, according to a senior Turkish official.

The official said Turkey was in contact with Russia, the United Nations and third parties regarding the issue.

The government of Ukraine has accused Russia of stealing grain from the territories that Russian forces have seized since their invasion began in late February. Russia has previously denied stealing any Ukrainian grain.