According to Maxar Technologies, which shared the high-resolution images, the development was reported near the Siret border crossing on Friday, NDTV reported.
Ukrainian nationals poured in large numbers into Poland, Romania, Hungary and Slovakia on Friday as Russian missiles pounded the capital Kiev, with many waiting for hours at congested border crossings.
A majority of those crossing into neighboring countries are reported to be women and children since Ukraine has restricted passage for men between 18 and 60 years old, news agency Reuters reported.
On the border with northern Romania, long queues were reported on Thursday as cars waited to board a ferry over the Danube river into Isaccea, a town between Moldova and the Black Sea.
At least 29,000 people had entered Poland from Ukraine on Thursday, though it was unclear how many were war refugees and not foreigners going home.
Activists have set up food and hot drink distribution points.
Bulgaria, meanwhile, has started issuing passports to its citizens in Kiev who needed travel documents and had sent four buses to the Ukrainian capital to evacuate people.
Three buses, evacuating some 130 Bulgarians will leave Odessa on Friday, the foreign ministry spokesman said. Some 250,000 ethnic Bulgarians live in Ukraine.
At least 1,000 Russian soldiers had been killed so far, Ukraine has said. Russia did not release casualty figures. The UN said 25 civilians had been killed and 102 wounded, figures that were likely to be a "significant under-estimate". None of the tolls could be independently verified.