"Implementation is going to be key and very challenging," Ashton told reporters in London. "Now what has to happen is the implementation, that begins with the withdrawal of any threat of violence, and that means anyone threatening violence."
Ukrainian President Viktor Yanukovich signed an agreement with three opposition leaders on Friday to end a crisis that sparked violent clashes between protesters and police on the streets of the capital Kiev.
The crisis began when Yanukovich rejected a wide-ranging free trade and association agreement with the European Union and instead turned to Russian President Vladimir Putin for support.
In a sign that the tug of war between East and West is far from over, Ashton said that the EU agreement was still a good one.
"The offer if you like, the agreement we reached, is a good one. We would like the country to go forward. President Yanukovich told me many times he would like to see that as well and I hope that that's what will happen."
She said that in November she had spoken for many hours to Yanukovich who had many questions about economic challenges: "That was the reason that he felt unsure about moving forward. We can answer all of those."
Ashton said she had spoken to Russian Foreign Minister Sergei Lavrov and the two had agreed that the violence in Ukraine must stop.
"We've been talking about the situation and we both agree it's very important that this violence stops and that we begin to see a move forward," Ashton said.
When asked whether the EU would lift sanctions in light of the deal, she said it would depend on information from EU foreign ministers on the ground in Ukraine.
"We will make the decisions in the light of what their information is telling us. It is important to synchronise this properly," she said.