The sanctions began on midnight after representatives of ECOWAS, the junta and other stakeholders failed to reached an agreement on Sunday in the Gambian capital Banjul.
The meeting was held after the junta last week accepted the deployment of a 600-strong ECOWAS mission, which is to replace 200 Angolan troops stationed in Guinea-Bissau to help with military reform.
ECOWAS had previously rejected a two-year transition agreed upon by the junta and some of the opposition parties.
The junta launched the coup on April 12 and arrested interim President Raimundo Pereira and ex-prime minister Carlos Domingos Gomes Junior, who were released and arrived in Abidjan, Cote d'Ivoire on Friday.
Guinea-Bissau is the second ECOWAS member state hit by a military coup in the past two months. The military in Mali seized power on March 22 and surrendered power this month to a transitional government under the pressure of ECOWAS. But the junta in Mali has rejected the deployment of ECOWAS troops.