Foreign institutional investors (FIIs) are back as the driving force of Indian stock market by pumping in over $1 billion in past month, paving the way for the benchmark Sensex once again to inch towards the 12,000 points milestone.Market analysts expect fervent market sentiments to sustain over the next week on the back of both foreign as well as domestic funds showing signs of a revival in buying activities.After a gain of more than 200 points over the past week, the bulls are betting high on the prospects of 30-share sensitive index returning above the psychological 12,000 mark.This would be as a result of good buying support from both FIIs and mutual funds, as well as other factors like good auto sales and the government decision to keep fuel prices unchanged.The Sensex ended the last week's trading at 11,778.02, which was at a gettable 222 points away from the 12K-level.FIIs, who have been among the major drivers of the Indian stock market during the Sensex journey from 7,000 level in June 2005 to 12,000 in April this year, purchased shares worth Rs 4,774 crore (over $1 billion) in the month of August, which is the biggest one-month net purchase in the last five months.Meanwhile, 22 new FIIs entered Indian market in August - the highest number in the past four months. The market observers expect the upsurge in FII inflows as well as their numbers to reignite the market sentiments after a lull during April-July period.