Authorities have arrested five Nigeriens at a port in Benin, accusing them of entering the site fraudulently, the latest in a tit-for-tat row between the West African nations.
The Nigeriens were arrested late on Wednesday at the port of Seme-Kpodji, which has become a flashpoint in relations between the neighbours.
Landlocked Niger runs a pipeline from its oilfields to the port on the Atlantic coast to export its crude.
Under regional sanctions imposed on Niger after a July coup, Benin closed the border but has since reopened it.
Niger's military rulers however have refused to reopen their side.
Benin announced on national television on Wednesday that it was lifting a "blockade" on Nigerien oil, saying "the tap on the Benin-Niger pipeline is unblocked again".
Prosecutors also announced that five Nigeriens had been arrested at the port and accused at least two of being "agents" of the Niger junta.
"To justify this fraudulent entry onto the site, the interested parties indicated that they were all employees of WAPCO Niger whose badges they displayed," financial crimes and terrorism prosecutor Elonm Mario Metonou said in a statement.
But "at least two of these people are Nigerien agents in the service of the National Council for the Safeguard of the Homeland (CNSP)" he said.
They "had fake WAPCO Niger employee badges made for the occasion", the prosecutor added, referring to the Chinese company that operates the pipeline.
An investigation is ongoing, the statement said.
Niger says those arrested are the "deputy general director of WAPCO" Ibra Hadiza and "four engineers on a supervisory mission".
Oil Minister Mahamane Moustapha Barke accused Benin of "violating agreements" on the transport of Niger's crude via the Beninese port.
He said that under the accords, loading must be carried out in the presence of Beninese, Nigerien and Chinese officials.
But, he added, Benin authorities had "decided to prevent the Nigerien party from accessing the installations".
"We cannot accept that these loadings take place under these conditions," he said.
Niger's military leader General Abdourahamane Tiani has given instructions to stop the Koulele pumping station in northeastern Niger if the five are not released, he said.
Niger accuses Benin of hosting "French bases" in the north of the country, with the aim of "training terrorists who must come and destabilise our country".
Benin and France refute the accusations.
The closure of the border with Niger hurt Benin's public revenues and increased the cost of food, prompting labour union protests over the high cost of living.