The monsoon has delivered 38% lower-than normal rainfall since the start of the season on June 1. (FILE)

India's annual monsoon rains have covered nearly half of the country and conditions are favourable for further advancement into the central and western parts this week, a weather department official said on Monday.

The monsoon's progress will help farmers to accelerate sowing of summer-sown crops, which has been lagging due to a delay in the arrival of monsoon rains.

"In the last three days, monsoon has gained momentum. It has covered the entire southern and eastern India," said an official at the India Meteorological Department (IMD), who declined to be named.

Conditions are favourable for further advancement of monsoon rains into Madhya Pradesh and Gujarat, the official said.

The monsoon arrived in Kerala on June 8 versus the usual June 1. However, Cyclone Vayu developed in the Arabian Sea drew moisture from the monsoon and weakened its progress. The monsoon typically covers half of India by mid-June.

The monsoon has delivered 38% lower-than-normal rainfall since the start of the season on June 1, due to a delay in the onset of monsoon rains, according to data compiled by IMD.

"The rainfall deficit has come down to 38% from last week's 44%. By the end of the month, it would be much lower," the official said.

Monsoons deliver about 70% of India's annual rainfall and are the lifeblood of its $2.5 trillion economy, spurring farm output and boosting rural spending on items ranging from gold to cars, motorcycles and refrigerators.

Sowing of summer-sown crops such as cotton, rice, soybean, corn and pulses has been delayed by a fortnight, but could accelerate in coming weeks due to the recent spell of rainfall, said Harish Galipelli, head of commodities and currencies at Inditrade Derivatives & Commodities in Mumbai.

Farmers have planted summer-sown crops on 9.1 million hectares as on June 21, down 12.5% compared with the same period a year ago, according to provisional data from the Ministry of Agriculture. Cotton sowing was down 12%, while soybean planting has lagged by 57% during the period.

"The sowing delay wont impact on yields if we get good rainfall in the next two months," Galipelli said.

India is the world's biggest exporter of rice and top importer of edible oils. A drop in oilseeds and rice production due to poor monsoon could lead to lower rice exports and lift the country's imports of edible oil such as palm oil, soyoil and sunflower oil, dealers said.