Builder demanding extra money before handing over flat, is it legal?

I purchased a flat in Noida from XYZ Builders and paid the entire amount mentioned in the agreement. Now, just before registration, they are demanding an additional ₹3 lakh, citing increased costs. They are refusing to hand over the keys until I pay. I feel cheated and am not sure if this demand is legal. The agreement has no mention of any such extra charges. What legal action can I take to get possession of my flat?