"We will enquire the issue. We will call everyone (all parties) in the future," Home Minister Sushilkumar Shinde said in the Lok Sabha.
However, he noted that all parties were invited for today's meeting.
Six regional and small parties took "strong exception" to not being invited for the meeting and asked Prime Minister Manmohan Singh to ensure that all parties are invited at these crucial meetings in future.
"Lately, such meetings are being called selectively inviting only selected leaders, leaving out important parties, including national parties and such meetings are construed as all-party meetings. This is very unfair and unfortunate. We take strong exception to this practice," they said in the letter.
They urged the Prime Minister to "see that such meetings are called giving representation to all the parties with representation in Parliament."
The signatories included Gurudas Dasgupta (CPI), Anant Geete (Shiv Sena), B Mahtab (BJD), S Semmalai (AIADMK), Nama Nageshwar Rao (TDP) and Rattan Singh Ajnala (Akali Dal).