Bavaria on Tuesday threatened Chancellor Angela Merkel's federal government with a lawsuit if it doesn't take measures to further secure the German border and reduce the influx of asylum-seekers, escalating a long-running dispute over refugee policy.

Bavarian governor Horst Seehofer sent a letter to Merkel setting out demands such as "effective border controls" at all crossings and an annual cap of 200,000 refugees for Germany, the state government said.

Last year, Germany registered nearly 1.1 million asylum-seekers entering the country.

The demands, and the threat to file a lawsuit with the Federal Constitutional Court if Berlin doesn't satisfy the German state that has been on the front line of the surge of migrants, weren't in themselves new. However, Bavaria's drafting of a formal letter to the federal government increased the tensions in Merkel's governing coalition.

Bavaria — where most migrants first enter Germany — is run by the Christian Social Union, the regional sister party to Merkel's conservatives and itself part of Germany's governing coalition. Seehofer has been at odds with Merkel for months over her open-door refugee policy and has pressed increasingly loudly for a cap on numbers.

There's wide agreement in Merkel's coalition of right and left that Germany must get its refugee numbers down, but festering disagreement about how to do it. Merkel argues that diplomacy with Germany's European partners, Turkey and others is the key to resolving the crisis, and that unilateral measures such as a cap aren't feasible.

"Bavaria needs effective measures immediately" in the absence of a fast European solution, the state's interior minister, Joachim Herrmann, said in a statement. Seehofer didn't say at what point Bavaria might actually file a lawsuit.

Bavaria didn't immediately release the letter and the federal government had no immediate comment on it. Merkel's and Seehofer's center-left coalition allies, however, blasted the state's move as "absolutely incomprehensible and absolutely inappropriate" and cautioned the CSU against endangering the ruling coalition.

"The CSU must decide whether it wants to be in the government or the opposition," said the Social Democrats' caucus leader, Thomas Oppermann. "In the government, we have to work with each other and not against each other."