India's crucial monsoon rains are slightly delayed but not yet a cause for concern, the head of the weather office said on Thursday.

"Overall monsoon progress is slightly behind schedule but such delays are usual," L.S. Rathore, director-general of the India Meteorological Department (IMD), told journalists, adding there was no cause for concern yet.

He added that India was likely to give its second official monsoon forecast on Friday.

The June-September rains arrived over the southern Kerala coast four days behind the usual start date of June 1. Rains are needed for about 55 percent of India's agricultural land which does not have irrigation.

There will be a weekly weather update later on Thursday.