The Ethiopians withdrew as part of a peace deal
Ethiopian troops have re-entered Somalia just two weeks after pulling out, according to witnesses.
Local officials said Ethiopian soldiers had set up a checkpoint in Hiran region of central Somalia, some 20km (12 miles) from the border.
The Ethiopian government described the reports as false and said it had no intention of returning to Somalia.
Islamist militias run much of central Somalia and some are loyal to the new President, Sheikh Sharif Ahmed.
Ethiopian troops occupied parts of Somalia for two years after ousting the Union of Islamic Courts (UIC) from the capital Mogadishu.
The Ethiopian withdrawal was part of a peace deal agreed recently between the government and moderate Islamists.
They left behind African Union peacekeeping troops and Somali government soldiers in Mogadishu but analysts have said that force is unlikely to be able to keep the advancing Islamists at bay.
Al-Shabab fighters quickly seized more territory as the Ethiopians pulled out
"The Ethiopian forces are violating the basic integrity of Somalia again and they entered the Hiran region only days after their government announced its complete withdrawal from the country," UIC commander Ahmed Osman Abdalla told AFP news agency.
Addis Ababa has said it is keeping a heavy troop presence on the border in case of threats to its security.
But Information Minister Bereket Simon called the report that Ethiopian troops had crossed back into Somalia a "wicked" distraction.
He told Reuters news agency: "The army is within the Ethiopian border. There is no intention to go back."
Hardline Islamist militia al-Shabab, which is labelled a terrorist organisation by the US, took advantage of Ethiopia's pull-out from Somalia to boost its control of the south.
Its fighters last week grabbed Baidoa, the seat of the Somali parliament, on the same day that Ethiopia said its soldiers had finished their withdrawal.
Al-Shabab has been holding demonstrations this week against Somalia's new president, a moderate Islamist whom the radicals accuse of selling out to the West.
Mr Ahmed was elected at the weekend as part of a UN-brokered plan to try to form a unity government and bring peace to Somalia for the first time since 1991.