The three media organizations are among the world's leading suppliers of financial news and information. They compete to cover news in real time from events such as OPEC+ meetings, which can have a material impact on the price of oil and the global cost of energy.

The Organization of the Petroleum Exporting Countries and its allies, known as OPEC+, includes top oil producers Saudi Arabia and Russia. OPEC+ pumps more than 40% of the world's oil supply.

OPEC declined to comment on why reporters from the three media organizations were not invited to cover the OPEC-hosted July 5-6 seminar in Vienna.

"We believe that transparency and a free press serve both readers, markets and the public interest, and we object to this restriction on coverage," a spokesperson for Reuters, the news and media division of Thomson Reuters Corp (TRI.TO), said on Wednesday.

"Reuters will continue to cover OPEC in an independent, impartial and reliable way, in keeping with the Thomson Reuters Trust Principles."

Reporters at Reuters received an email on Tuesday stating that earlier accreditation was not an invitation to attend. Bloomberg and the Wall Street Journal reporters received a similar communication, sources familiar with the matter said.

"We are very concerned by the prospect of OPEC excluding certain journalists, including from Bloomberg, from next week's seminar," Bloomberg News said in a statement.

"For the sake of market transparency, we strongly advocate for OPEC to allow journalists from relevant global news outlets to attend."

The Wall Street Journal did not respond to a request for comment.

This would be the second consecutive OPEC+ event in which OPEC has restricted media coverage. The same media groups were denied access to OPEC's Vienna headquarters during a June 4 oil policy meeting. OPEC gave no reason for excluding the three organizations from the previous policy meeting in June.

The email from OPEC's public relations department to Reuters reporters on Tuesday stated press participation was by invitation only and that earlier accreditation - which OPEC called "self accreditation"- would not grant access to the event.

OPEC had sent emails earlier in June to all members of the Reuters team who had applied for accreditation, saying their registration had been completed and that OPEC was looking forward to welcoming them to the seminar. The email included a QR code to be presented with ID at in-person registration for the event.

OPEC sent an email on Tuesday inviting reporters at other media organizations to attend, the sources said. Those included the Financial Times and trade publication Argus, as well as S&P Global Commodity Insights, known as Platts, the sources said. The communication stated "this email serves as your personal invitation," according to a copy forwarded to Reuters.

Argus confirmed it had been invited and would attend. The Financial Times declined to comment and Platts did not respond to a request for comment.