The United States has begun training more than 50 Ukrainian military service members on the howitzers Washington is providing, according to a senior American defense official.

The training is expected to take approximately a week, the official told reporters on Wednesday, while the Pentagon has been tight-lipped about where it’s happening. Pentagon spokesman John Kirby said on Monday the training would take place “outside” of Ukraine. The Ukrainian soldiers will then return to Ukraine, where they will train additional soldiers.