Growth in the sector, which has a combined weight of about 38 per cent in the Index of Industrial Production (IIP), stood at 2.3 per cent in May against 4.2 per cent in April.
"The regulatory issues linked to the fixing of gas price have been affecting the performance of oil and gas sector," Assocham Secretary General D S Rawat said.
On the steel sector's poor performance, Rawat said it has been affected by steel imports mainly from Korea and Japan at reduced tariffs prescribed in free trade agreements (FTAs).
However, according to the data released by the Ministry of Commerce and Industry, the output of coal, fertilisers, cement and electricity went up by 5.5 per cent, 17.6 per cent, 8.7 per cent and 6.3 per cent, respectively.
During April-May, growth in the eight core industries slowed to 3.3 per cent as against 4.9 per cent in the year-ago period.