Ashton said in an interview with The Wall Street Journal that the package would be aimed at helping Ukraine get through a transition period during which a broad-based interim government could approve political and economic reforms and prepare for presidential elections, currently due next year.
The EU and the United States are "developing a plan - a Ukrainian Plan, I have suggested they call it - that looks at what do we need to do in different parts of the economy right now to make things better," the paper quoted Ashton as saying on the sidelines of the Munich Security Conference.
Ukraine has been gripped by mass protests that have turned violent at times since President Viktor Yanukovich spurned a trade and cooperation agreement with the European Union last November in favour of closer ties with Russia (see background).
Ukraine won a $15 billion (€11 billion) bailout pledge from Russia to help its ailing economy. But after Ukraine's Prime Minister Mykola Azarov quit last week, bowing to protesters' demands, Russian President Vladimir Putin said Moscow would wait until a new government was formed before fully implementing the agreement.
Ashton said the amount of the proposed EU-US aid package had not yet been decided but said "the figures won't be small".
She said the package might not only consist of money but could also contain "guarantees" or help on investment or shoring up the currency, though she did not elaborate.
Unlike past EU aid promises, the package would not depend on Ukraine reaching an agreement with the International Monetary Fund, Ashton was quoted as saying.
Ashton visited Ukraine last week to try to promote dialogue between the government and opposition and she is expected to return to Kiev on Tuesday. US Assistant Secretary of State Victoria Nuland will also visit Ukraine again in the next few days.
Opposition leaders, who addressed a crowd packing Kyiv's Independence Square on Sunday, spoke of international mediation.
Arseny Yatsenyuk, leader of Tymoshenko’s Batkyvshchyna party, was quoted as saying by the Ukrainian media that according to the opposition, the minimum amount of EU-U.S. aid should be $15 billion (€11 billion).
In a video distributed by the US embassy, Obama called for a democratic process to produce a government with "greater legitimacy and unity". "That's going to be challenging but we're trying to help on the negotiations on that," he said.
Yanukovich to return from ‘sick leave’
In the meantime, the Ukrainian government bowed to intense Western pressure to let an opposition activist fly abroad for treatment after his abduction, torture and then attempted arrest by police outraged critics of President Viktor Yanukovich.
The embattled head of state, caught in a tug of war between Russia and the West and facing mass protests that have prompted fears of civil war, announced he would return from four days of sick leave today (3 February). It was unclear whether he might resume hesitant moves toward compromise or hit back at his opponents.
Yanukovich retains the loyalty of a substantial section of the 45 million population, notably among Russian speakers in the east. A weekend poll showed he would top a multi-candidate presidential election with about 20% of the vote.
In the meantime, Dmytro Bulatov, 35, whose bloodied face and account of being "crucified" during a week in the hands of mysterious kidnappers has dominated opposition media since Friday, flew to EU state Lithuania. Intense negotiations with Western diplomats had led to a court lifting a charge of "mass disorder" against him, on which police had tried to arrest him at a Kyiv clinic.
"Without the support of the European community this would not have happened," former world champion boxer and opposition leader Vitaly Klitschko told Reuters. "International pressure gives us a chance to fight for freedom in Ukraine."
Western officials, some of whom met Klitschko and other opposition leaders at an international security conference in Munich over the weekend, have been trying to push the rival sides toward compromise, diplomats say.
Russian officials, too, while condemning what one senior parliamentarian called "aggressive colonists" from the EU, have also spoken of a role in promoting a peaceful settlement.
Klitschko, a former world champion heavyweight boxer, told supporters at the Kyiv rally that he and other opposition leaders discussed with Western officials in Munich bringing in international mediators to smoothe talks with the president.
Yatsenyuk called on the authorities to free 116 prisoners, as well as the leader of his party, ailing former prime minister Yulia Tymoshenko, whose freedom has also been a demand of the European Union since she was jailed in 2011.
Opposition leaders said they had assurances of economic support from the West if they could take power from Yanukovich and spoke of new elections and amendments to the constitution that would curb the presidency and bolster parliament. Yanukovich supporters say that would mean political paralysis.
Some opposition leaders have urged the EU to impose sanctions to hurt the business and financial interests of the president and his leading supporters. However, few Western governments see that as worthwhile at present.
The deputy head of Tymoshenko’s Batkyvshchyna party Hryhoriy Nemyria was quoted saying ty the Ukrainian website Leviy bereg that Swedish foreign minister Carl Bildt told him that sanction against Ukraine leaders and oligarchs would be discussed by EU ministers on 10 February.