He said the states will be closely monitored till April 20.
If in some regions, it is found that the pandemic has almost no impact, some relaxations will be given after due consultation.
The fight against coronavirus will be made more stringent in the next week, the Prime Minister said, adding the next lockdown will be stricter.
Modi said suggestions from various stakeholders including states and people have favoured extension of lockdown.
In his address, Modi thanked the countrymen in abiding by the guidelines of the lockdown.
He said because of the lockdown and social distancing India could manage the pandemic much better than most of the developed countries.
He said India took the bold step of announcing 21-day lockdown when there were only 550 coronavirus cases.
He said, the country did not wait for the pandemic to spread to take drastic steps.
He said if India did not take a holistic and integrated efforts, the situation would have blown out of proportion.
He also said the country had to pay a heavy price as far economy is concerned, the lockdown had a telling effect on the overall health scenario.
Modi also thanked the states for their efforts in containing the deadly virus.
Earlier, in the wake of COVID-19 pandemic and with the aim to prevent the spread of coronavirus, Modi had announced the 21-day nationwide lockdown on March 24 last that began from the next day.
The 21-day country-wide lockdown was scheduled to end on Tuesday midnight.
The Prime Minister last week held a meeting with all the chief ministers of the states, in which he was urged by almost all to extend the lockdown by at least 15 days.
Several states including Punjab, Odisha, Maharashtra, and West Bengal have already extended the lockdown till April 30.
Mizoram became the first north-eastern state to extend lockdown till April 30.
Meanwhile, Meghalaya went for a 48-hour curfew in capital city Shillong after the State registered the first COVID-19 case on Monday.
The COVID-19 positive patient is a 69-year-old doctor of Bethany Hospital in Shillong.
Confirming the news, Meghalaya health minister AL Hek on Monday said the doctor tested positive for the deadly COVID19 virus.
Hek said Bethany Hospital has been sealed indefinitely.
Meanwhile, the country has witnessed a surge in coronavirus cases over the last one week.
The number of positive cases in the country is about to touch the 10,000 mark with over 300 deaths.
According to the ICMR, a total of 2,17,554 samples from 2,02,551 individuals have been tested in the country as of 9 pm on April 13.
Assam has recorded the maximum number of COVID-19 cases with figures touching 31.
Arunachal Pradesh, Manipur, Mizoram and Tripura had earlier recorded one case each.
Sikkim is now the only state in the Northeast which is free from COVID-19 cases.