The state government had imposed the curfew on March 24.
In a video statement, Thakur said Prime Minister Narendra Modi had decided to extend the lockdown beyond May 17 after talking to chief ministers of various states.
The fourth phase of the lockdown will continue till May 31, the chief minister said.
Noting that coronavirus cases had increased after the return of stranded people from other states, the chief minister said that the number of cases may rise further as about 60,000 more people had expressed their wish to come back to Himachal Pradesh.
At the same time, he said '"there is no need to worry".
"Himachal Pradesh is still better than other states," he said and urged the returnees to follow quarantine norms whether they were being kept in any institution or at home.
He said the curfew will remain imposed till May 31 to check the spread of coronavirus.
Thakur said "the nation is fighting the COVID-19 pandemic under the able leadership of Prime Minister Narendra Modi" and thanked the PM for announcing a Rs 20-lakh-crore package for the revival of economy.
Stating that two 'corona warriors' also tested positive for COVID-19 in Kangra a few days ago, he wished them speedy recovery.
Himachal Pradesh has reported 81 coronavirus cases and three deaths. DJI RTRT