"Our detainees for theirs," one of them said in an interview on Al-Jazeera television, adding that his group has "contacted our leadership in Mali."
The kidnapper, identified as Abu al-Baraa, as well as three of the hostages, also demanded the withdrawal of the Algerian army which has surrounded the gas field, so that negotiations might begin.
"We demand the Algerian army pull out from the area to allow negotiations," he told the Doha-based satellite channel.
Abu al-Baraa also told the channel that Algerian snipers had opened fire at the site where the hostages are being held, injuring a foreigner he identified as a Japanese national.
The kidnapper confirmed earlier reports that there are "around 41" hostages from several countries: France, Norway, the United States, Britain, Romania, Colombia, Thailand, the Philippines, Ireland, Japan, South Korea, and Armenia.
A Briton, a Japanese and an Irishman, identified as being among the hostages, took turns speaking on Al-Jazeera to demand the withdrawal of Algerian troops.
The three men, who spoke in turn by telephone, urged the army to stop opening fire at their place of detention and to preserve their lives and encourage negotiations.
The hostages spoke in English but their remarks were voiced over in Arabic by Al-Jazeera. They echoed Abu al-Baraa's calls for the withdrawal of the Algerian army.
A man identified as an Irish hostage said that the situation was deteriorating and demanded Algeria's army pull out and halt its fire on the camp.
He also said that he had contacted his wife earlier. The other two hostages made similar appeals.
Islamist gunmen killed two people and took 41 foreign hostages in the attack on Wednesday at the In Amenas gas field near the Libyan border.
They said the attack was to avenge the Algerian government's decision to open its airspace to French warplanes pounding Islamist rebels in neighbouring Mali.
Algeria on Wednesday sent troops to the site, and insisted it would not negotiate with the gunmen.