India's crucial monsoon rains should pick up in July after a slow start over vast swathes of the country, which has threatened crops from rice to sugar, forecasters said.

Some 26 out of India's 36 weather zones received "deficient" or "scanty" rains in the past week from the monsoon which typically sweeps the subcontinent from June to September, according to the weather office's website on Saturday.

"The monsoon rains are expected to pick up in the latter half of next week," Swati Basu, acting Director General of the India Meteorological Department (IMD), told the semi-official news agency Press Trust of India late on Friday.

Basu forecast good rains in July which along with August are key months for planting and when India usually receives the maximum amount of rain.

For the past week, monsoon rains were 18 percent below average while for June as a whole they have been 23 percent below average -- fanning worried murmurs about a repeat of a drought that devastated Indian farmers in 2009.

"The monsoon has definitely started off on a sour note," economist Indranil Pan from the Indian investment house Kotak said in a note to clients.

The monsoon is dubbed an "economic lifeline" in the country of 1.2 billion people that is one of the world's leading producers of rice, sugar, wheat and cotton.

India's 235 million farmers still rely on the erratic rains to soak around 60 percent of the country's farmland -- despite calls for the government to improve irrigation and water-harvesting methods to ensure more stable crop output.

The rains have been abundant in India's northeast where raging floods have forced hundreds of thousands of people to flee homes but weak in the vital northwestern grain bowl and oil seed-growing central regions.

"Meteorological conditions indicate an increase in rainfall activity over east, central and also over northwest India" in coming days, said the weather office forecast.

But still, authorities have told India's 29 states to draft plans to shift from thirsty crops such as rice and sugarcane to others such as beans and wheat which require less water if the monsoon does not become stronger by mid-July.

The uncertain start to the rainy season means more worries for the beleaguered Congress government, already buffeted by corruption scandals and an economy growing at its slowest pace in nine years.

The monsoon season "will be crucial given the existing challenges to the economy," said Kotak's Pan.

Farming's contribution to India's gross domestic product has fallen from 50 percent in the 1950s to around 15 percent. But it remains vital to the economy by supporting 700 million rural Indians and fuels demand for everything from TVs and refrigerators to motorcycles and gold.

India this month said it would maintain its forecast for an average monsoon.

But the meteorological department was spectacularly wrong in its forecast in 2009 when it predicted a normal monsoon and the country suffered its worst drought in 37 years.

The drought sent food prices rocketing, causing huge hardship for the country's hundreds of millions of poor.