The African nations also threatened that if the EU does not back their case, they would oppose Europe's proposal to ban trade in the giant fish.
A nine-year ban on ivory sales was agreed in 2007 under the Convention on International Trade in Endangered Species (CITES), but two African nations - Tanzania and Zambia - want to reclassify their elephant populations as a first step to resuming the trade, a leaked letter from the 23 countries says.
The group of African countries, which includes Kenya, Ethiopia and Nigeria, are concerned that most EU countries support Tanzania and Zambia's attempts to restart the ivory trade, the letter says.
EU ambassadors met on Friday to finalise the bloc's position at the next CITES meeting which starts on March 13.
The diplomats are expected to confirm support for an endangered listing for the Atlantic bluefin, which would effectively ban trade in the endangered fish which can fetch up to $100,000 each at market.
"Please do not force our collective hand to cast our 23 votes against the EU on any of the issues it is supporting such as, for example, the high-profile proposed ban on bluefin tuna," said the letter seen by Reuters.
After lengthy talks on Friday morning on bluefin tuna, ivory and polar bears, the bloc could struggle to reach a unified position by 13 March, an EU source said.
Poachers in many central and west African countries continue to kill elephants for their ivory, which is used for trinkets and also as an aphrodisiac in countries such as China.
The legal ivory trade fuels and provides cover for unlawful sales, the 23 African countries argue.
(EurActiv with Reuters.)