NATO chief Jens Stoltenberg warned Turkey on Thursday it must fully respect legal due process, as Ankara continues to make mass arrests after last July's failed coup.
"Turkey has the right to protect itself and to prosecute those who were behind the failed coup attempt but that has to take place based on the full respect of the rule of law," Stoltenberg said as he arrived for an EU defence ministers meeting in Valletta.
"I attach a great importance to these values myself and this is an issue we have discussed with the Turkish leadership," he added.
Turkey on Wednesday detained more than 1,000 people and suspended over 9,000 police in a vast new crackdown against alleged supporters of Fethullah Gulen, the US-based preacher President Recep Tayyip Erdogan accuses of mounting the coup.
Erdogan earlier this month narrowly won a controversial referendum which gives him increased powers, sparking increased concerns in Europe over the country's rights record.
But at the same time, Turkey is a crucial NATO ally, sitting at a strategic crossroads and providing the US-led alliance with its second biggest military component after the United States.
"Turkey is a key ally for many reasons, especially for its strategic geographic location bordering Iraq, Syria and all the violence, the turmoil we see to the south but (it is) also close to Russia and the Black Sea," Stoltenberg noted.
The NATO secretary general, a former Norwegian prime minister, said he looked forward to an alliance summit in May in Brussels where all 28 member state leaders will be able to "discuss our common challenges."