"The Russian president once said that if a fight is inevitable, you need to hit first. But every leader needs to understand that a fight must not be inevitable when it concerns a real war and millions of human lives."
Navalny, 44, a prominent opponent of Russian President Vladimir Putin, started refusing food on March 31 in protest at what he said was the refusal of prison authorities to treat him properly for acute back and leg pain.
"His condition is indeed critical," said Alexandra Zakharova, a representative of the Doctors Alliance trade union - a group that Russian authorities regard as opposition activists.
There is a strong push in the United States to cripple the project with sanctions and prevent its completion, out of fear that it will give Moscow a dangerous grip on the energy supplies of US allies in Europe.
Ukraine has raised the alarm over an increase in Russian forces near its eastern border as violence has risen along the line of contact separating its troops from Russia-backed separatists in its Donbass region.
"The Chancellor demanded that this build-up be unwound in order to de-escalate the situation," Germany's government said in a readout of a telephone call between Merkel and Putin.