London (CNN) -- World powers convening Friday in Tunisia on the Syria crisis plan to forge a united plan on delivering humanitarian aid and develop a closer relationship with a nascent Syrian political opposition group.
"I think that the strategy followed by the Syrians and their allies is one that can't stand the test of legitimacy or even brutality for any length of time. There will be increasingly capable opposition forces," Secretary of State Hillary Clinton told reporters in London.
"They will find somewhere, somehow the means to defend themselves as well as begin offensive measures and the pressure will build on Russia and China. World opinion is not going to stand idly by," Clinton said.
"Arab opinion is not going to be satisfied watching two nations -- one for commercial reasons, one for commercial and ideological reasons -- bolster a regime that is, you know, defying every rule of modern international norms."
Thousands have died in Syria since Syrian President Bashar al-Assad's regime launched a crackdown against protesters nearly a year ago, and Al-Assad faces strong international pressure to end the bloodshed.
"We think the pressure will continue to build. It's a fluid situation, but if I was a betting person for the medium term and certainly the long term, I would be betting against" al-Assad, Clinton said.
Senior U.S. officials told CNN on Thursday that countries met in advance of the conference known as the "Friends of Syria" to hammer out a framework for delivering food and medicine.
Also, Clinton said the opposition Syrian National Council is emerging as an alternative to al-Assad's regime and that the consensus opinion among Arab League and other nations is that the group "is a credible representative of the Syrian people."
Diplomats said that view will be reflected in a communique to be issued from Friday's conference in which more than 70 countries have been invited to participate.
It is part of "ongoing efforts with our friends, allies, and the Syrian opposition to crystallize next steps to halt the slaughter of the Syrian people and pursue a transition to democracy in Syria," U.S. State Department spokeswoman Victoria Nuland said this week.
The Friends of Syria conference grew out of the unsuccessful attempts by the U.N. Security Council to adopt a resolution condemning the violence in Syria. Russia, a Soviet-era ally and arms dealer to Syria, and China vetoed that resolution. Neither country is taking part in Friday's conference.
Diplomats met on the sidelines of a London conference Thursday on rebuilding Somalia to discuss Syria. They included officials from the United States, Qatar, Saudi Arabia, France, Britain, Germany, Jordan and Morocco.
The international community has been incensed with Syria's crackdown on protesters, but it has been unable to stop the violence engulfing the country.
Countries plan to forge a comprehensive plan to deliver aid to Syria and press the regime to provide access within days, U.S. officials said.
Clinton has had discussions with other diplomats about how to help people stuck in "horrific conditions," such as the besieged city of Homs, a senior State Department official said.
Countries are shooting for "concrete proposals" on how the international community coordinates and plans aid, the official said.
"All of us have been working with various humanitarian organizations, U.N. organizations on the ground," the official said. "The real challenge is the access issue."
Clinton said that "we see a lot of developments that we think are pointing to pressure on (al-Assad). We hope it will pressure him to make the right decision regarding humanitarian assistance, but in the event that he continues to refuse, we think that the pressure will continue to build."
The opposition Syrian National Council has been working to reach out to people inside Syria to make sure that what they say Friday accurately reflects the reality on the ground inside the country.
A council delegation met with the International Committee of the Red Cross in Geneva, Switzerland, on Wednesday to discuss the humanitarian situation in Homs, bombarded daily by Syrian security forces.
"This meeting was held in response to the demands of the residents of Homs, in particular Baba Amr, to assess all the relief and humanitarian aid capabilities and options that might be provided to Homs residents, who have been suffering under the bombing and brutality of the (al-Assad) regime for weeks," the council said.
World powers also plan to meet with the Syrian National Council to discuss such plans for relief and a political transition.
One senior State Department official said he was "favorably impressed" with the Syrian National Council's increased outreach, given the complex political situation.
Countries looking to hear from the opposition group at the conference want to know its needs and then efficiently coordinate aid.
Officials also want to be assured the Syrian opposition has a credible plan to disprove al-Assad's assertion that the alternative to his regime would be chaos.
The Tunisia conference plans to talk about coordinating additional, stronger sanctions to increase pressure on al-Assad. The United States, the European Union, Turkey and the Arab League already have initiated sanctions.
Meanwhile, resistance fighters who've been operating in Syria under the banner of the Free Syrian Army may have some new hope, with some countries broaching the idea of arming rebel groups.
The United States called for more international action and hinted that arming the opposition isn't out of the question.
In Washington, White House press secretary Jay Carney said the conflict under al-Assad's regime demands reaction.
"We believe that we are in a situation where we -- the international community -- need to act in order to allow for the transition from (al-Assad) to a more democratic future for Syria to take place before the situation becomes too chaotic," Carney told reporters earlier this week.
Asked about calls by Sens. John McCain, R-Arizona, and Lindsey Graham, R-South Carolina, for the United States to consider arming the opposition, Carney said, "We don't want to take actions that would contribute to the further militarization of Syria, because that could take the country down a dangerous path.
"But we don't rule out additional measures that, working with our international partners, that the international community might take," he added.
CNN foreign affairs reporter Elise Labott reported from London. CNN's Joe Sterling reported from Atlanta. CNN's Steven Jiang reported from Beijing.